Samacheer Kalvi Class 9 Science Solutions Chapter 6 Light

I. Choose the Correct Answer:

 

Question 1. A ray of light passes from one medium to another medium. Refraction takes place when angle of incidence is
(a) 0°
(b) 45°
(c) 90°
(d) 60°
Answer: (c) 90°
In simple words: Refraction is the bending of light. When light hits a surface straight on (at 90 degrees), it passes through without bending. This is why 90 degrees is considered the angle for refraction to *not* take place, but rather direct transmission.

🎯 Exam Tip: Remember that "angle of incidence" is usually measured from the normal (a line perpendicular to the surface). If the angle of incidence *from the normal* is 0°, then the ray goes straight. If it is 90° *from the surface*, it's 0° from the normal.

 

Question 2. is used as reflectors in torchlight.
(a) Concave mirror
(b) Plane mirror
(c) Convex mirror
Answer: (a) Concave mirror
In simple words: Concave mirrors are good for torchlights because they can take light from a small bulb and spread it out into a strong, parallel beam. They collect and direct light very well.

🎯 Exam Tip: Concave mirrors are used when a parallel beam of light is needed, like in torches, or when a magnified image is desired, such as in shaving mirrors.

 

Question 3. We can create enlarged, virtual images with
(a) concave mirror
(b) plane mirror
(c) convex mirror
Answer: (a) concave mirror
In simple words: A concave mirror can make an object look bigger and appear behind the mirror, which is called a virtual image. This happens when the object is placed very close to the mirror, inside its focal point.

🎯 Exam Tip: To get an enlarged and virtual image from a concave mirror, the object must be positioned between the pole and the principal focus of the mirror.

 

Question 4. When the reflecting surface is curved outwards the mirror formed will be
(a) concave mirror
(b) convex mirror
(c) plane mirror
Answer: (b) convex mirror
In simple words: If a mirror's shiny side bulges outwards, like the back of a spoon, it is a convex mirror. This shape always makes things look smaller.

🎯 Exam Tip: Convex mirrors always form virtual, erect, and diminished images, regardless of the object's position.

 

Question 5. When a beam of white light passes through a prism it gets
(a) reflected
(b) only deviated
(c) deviated and dispersed
Answer: (c) deviated and dispersed
In simple words: When white light goes through a prism, it not only changes its path (deviates) but also splits into different colors (disperses) because each color bends a little differently. This is how rainbows are formed by water droplets acting as tiny prisms.

🎯 Exam Tip: The phenomenon of splitting white light into its constituent colors is known as dispersion, and it's a key property of prisms.

 

Question 6. The speed of light is maximum in
(a) vacuum
(b) glass
(c) diamond
Answer: (a) vacuum
In simple words: Light travels fastest when there's nothing in its way, like in outer space where there is a vacuum. In other materials like glass or diamond, light slows down.

🎯 Exam Tip: The speed of light in a vacuum is a universal constant, approximately \( 3 \times 10^8 \) meters per second, and is the maximum possible speed for any information or energy transfer.

 

II. State Whether True or False. If False, Correct the Statement:

 

Question 1. The angle of deviation depends on the refractive index of the glass.
Answer: True
In simple words: Yes, how much light bends when it goes through glass depends on how much the glass slows down light, which is its refractive index. More bending means a larger angle of deviation.

🎯 Exam Tip: A higher refractive index of the medium means light slows down more, leading to greater deviation and a larger bending angle.

 

Question 2. If a ray of light passes obliquely from one medium to another, it does not suffer any deviation.
Answer: False.
Correct statement: If a ray of light passes obliquely from one medium to another, it bends towards or away from the normal.
In simple words: When light moves at an angle from one material to another, it usually bends. This bending is called deviation. Only if it hits straight on (perpendicularly) will it not bend.

🎯 Exam Tip: Light changes direction (deviates) when it moves from one medium to another at an angle, because its speed changes. This is the definition of refraction.

 

Question 3. The convex mirror always produces a virtual, diminished and erect image of the object.
Answer: True.
In simple words: Convex mirrors always make images that look smaller, stand upright, and appear behind the mirror itself. These images cannot be caught on a screen.

🎯 Exam Tip: This consistent image formation property makes convex mirrors suitable for rearview mirrors in vehicles, as they provide a wider field of view.

 

Question 4. When an object is at the centre of curvature of concave mirror the image formed will be virtual and erect.
Answer: False.
Correct statement: When an object is at the centre of curvature of concave mirror the image formed will be real and inverted.
In simple words: If you place an object at the center of curvature of a concave mirror, the image will be real, upside-down, and the same size as the object, forming at the same point.

🎯 Exam Tip: For a concave mirror, an object placed at the centre of curvature (C) always produces a real, inverted image of the same size, also at C.

 

Question 5. The reason for brilliance of diamonds is total internal reflection of light.
Answer: True.
In simple words: Diamonds sparkle so much because their unique shape and material cause light to bounce around many times inside before it comes out. This is called total internal reflection, making them very brilliant.

🎯 Exam Tip: Diamonds have a very high refractive index and specific cuts that maximize total internal reflection, which is why they exhibit such strong brilliance and sparkle.

 

III. Fill in the Blanks:

 

Question 1. In going from a rarer to denser medium, the ray of light bends ............................
Answer: towards the normal
In simple words: When light moves from a less dense material to a more dense material, it slows down and bends closer to the normal line.

🎯 Exam Tip: Remember the phrase "TOWARDS the normal, when entering DENSER." This helps recall the direction of bending during refraction.

 

Question 2. The mirror used in search light is ......................................
Answer: concave
In simple words: Searchlights use concave mirrors to create powerful, focused beams of light. They gather light from a bulb and send it out in a straight, strong line.

🎯 Exam Tip: Concave mirrors are ideal for applications requiring parallel beams, like searchlights, as they focus light from a source placed at their focal point into parallel rays.

 

Question 3. The angle of deviation of light ray in a prism depends on the angle of .
Answer: incidence
In simple words: The amount that a light ray bends when passing through a prism depends on the angle at which the light first enters the prism.

🎯 Exam Tip: The angle of incidence is crucial for determining the path of light through a prism and the resulting deviation, following Snell's law.

 

Question 4. The radius of curvature of a concave mirror whose focal length is 5cm is ............................
Answer: 10 cm
In simple words: For any spherical mirror, the radius of curvature is always twice its focal length. So, if the focal length is 5 cm, the radius of curvature is 10 cm.

🎯 Exam Tip: The relationship \( R = 2f \) is fundamental for spherical mirrors; ensure you apply it correctly for both concave and convex mirrors.

 

Question 5. Large ............................ mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Answer: concave
In simple words: Big concave mirrors are used in solar furnaces because they can collect sunlight from a wide area and focus it into one small, very hot spot. This helps to create high temperatures needed for heating.

🎯 Exam Tip: Concave mirrors are converging mirrors, meaning they bring parallel rays of light to a single focal point, making them excellent for concentrating energy.

 

IV. Match the Following:

 

Answer: The correct matches are given in the table above.
In simple words: This table shows which scientific term or mirror type matches certain observations or uses in daily life. For example, magnification tells us how much bigger or smaller an image is, and convex mirrors help us see a wide area, like in mountain bends.

🎯 Exam Tip: Understand the basic properties and applications of different optical phenomena and devices to accurately match them with their descriptions or uses.

 

V. Assertion & Reason:

 

Question 1. Assertion: For observing the traffic at a hairpin bend in mountain paths a plane mirror is preferred over the convex mirror and concave mirror. Reason: A convex mirror has a much larger field of view than a plane mirror or a concave mirror.
(a) If both assertion and reason are true and reason is the correct explanation.
(b) If both assertion and reason are true and reason is not the correct explanation.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true
Answer: (d) Assertion is false but reason is true
In simple words: The statement that a plane mirror is preferred is wrong. However, the reason given, that a convex mirror shows a wider view, is correct. So, convex mirrors are actually better for seeing around bends.

🎯 Exam Tip: Convex mirrors are chosen for applications requiring a wide field of view because they form virtual, erect, and diminished images, effectively compressing a large area into a small image.

 

Question 2. Assertion: The incident ray is directed towards the centre of curvature of spherical mirror. After reflection it retraces its path. Reason: Angle of incidence (i) = Angle of reflection (r) = 0°.
(a) Both assertion and reason are true and reason is the correct explanation
(b) If both assertion and reason are true and reason is not the correct explanation.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true
Answer: (a) Both assertion and reason are true and reason is the correct explanation
In simple words: If a light ray aims for the center of curvature of a mirror, it bounces back along the same path. This happens because the ray hits the mirror surface at a 90-degree angle, making both the angle of incidence and reflection zero. This principle is used in many optical devices.

🎯 Exam Tip: Rays passing through or directed towards the centre of curvature always strike the mirror surface normally (perpendicularly), causing them to retrace their path after reflection.

 

VI. Answer Very Briefly:

 

Question 1. According to cartesian sign convention, which mirror and which lens has negative focal length?
Answer: A concave mirror and a concave lens have a negative focal length.
In simple words: When we use the standard way of measuring distances in optics, both concave mirrors and concave lenses are given a "minus" sign for their focal length. This is because their focal points are in front of the mirror or on the same side as the object for a lens.

🎯 Exam Tip: Remember that focal lengths are positive for converging optical elements (convex lens, concave mirror) and negative for diverging elements (concave lens, convex mirror) when the focus is considered relative to the direction of light propagation.

 

Question 2. Name the mirror(s) that can give (i) an erect and enlarged image, (ii) same sized, inverted image
Answer: A concave mirror can give (i) an erect and enlarged image (when the object is between the pole and focus), and (ii) a same-sized, inverted image (when the object is at the center of curvature).
In simple words: A concave mirror is special because it can make objects look bigger and upright, or the same size but upside down, depending on how far away the object is.

🎯 Exam Tip: It is important to know the six different image formation cases for concave mirrors, as they produce a wide variety of image types (real/virtual, erect/inverted, magnified/diminished/same size).

 

Question 3. If an object is placed at the focus of a concave mirror, where is the image formed?
Answer: The image will be formed at infinity as real and inverted. This happens because all rays from the focal point become parallel after reflection.
In simple words: When an object is exactly at the focus point of a concave mirror, the light rays bounce off and travel outwards in parallel lines, so they never meet to form an image at a specific point; it's like the image is infinitely far away and upside down.

🎯 Exam Tip: The principle of rays becoming parallel after reflecting from a concave mirror's focus is utilized in searchlights and headlights, where a light source is placed at the mirror's focus to produce a strong, parallel beam.

 

Question 4. Why does a ray of light bend when it travels from one medium to another?
Answer: A ray of light bends when it travels from one medium to another due to the change in velocity of light in the two different mediums. For instance, light slows down when moving from air to water.
In simple words: Light bends because its speed changes when it moves from one material to a different one. Imagine a car moving from a smooth road onto sand at an angle; it would change direction.

🎯 Exam Tip: The change in the speed of light is the fundamental cause of refraction. The extent of bending depends on the refractive indices of the two media and the angle of incidence.

 

Question 5. What is the speed of light in vacuum?
Answer: The speed of light in vacuum is known to be almost exactly \( 300,000 \) km per second. In 1665, the Danish astronomer Ole Roemer first estimated the speed of light by observing one of the twelve moons of the planet Jupiter, marking a significant early scientific measurement.
In simple words: Light travels incredibly fast in empty space, about 300,000 kilometers every second. That's fast enough to go around the Earth more than seven times in one second!

🎯 Exam Tip: The speed of light in a vacuum, often denoted by 'c', is a fundamental constant in physics, essential for calculations involving light, waves, and relativity.

 

Question 6. Concave mirrors are used by dentists to examine teeth. Why?
Answer: Concave mirrors are used by dentists because they produce virtual, erect, and magnified images when an object (like a tooth) is placed close to the mirror (between the pole and focal point). This allows the dentist to see a larger, upright view of the tooth. These mirrors are easy to sanitize and reuse.
In simple words: Dentists use concave mirrors so they can see your teeth up close and bigger. This helps them find any problems or cavities more easily.

🎯 Exam Tip: Always remember that concave mirrors are useful for magnification when the object is placed within the focal length, which is why they are often called "magnifying mirrors" in this context.

 

VII. Answer Briefly:

 

Question 1. a) Complete the diagram to show how a concave mirror forms the image of the object. b) What is the nature of the image?
Answer:
a) The ray diagram showing image formation by a concave mirror when the object is at C is given below.

b) Real, inverted and magnified. This is incorrect based on the diagram for object at C; for an object at C, the image is real, inverted, and of the same size. For an object between F and C, it is real, inverted, and magnified. The source text has inconsistencies here.
In simple words: When an object is placed at the center of curvature of a concave mirror, the reflected light rays meet to form an image at the same point. This image will be real, upside down, and the same size as the original object.

🎯 Exam Tip: When drawing ray diagrams, always use at least two standard rays (e.g., parallel to the principal axis reflecting through F, or passing through C retracing its path) to accurately locate the image.

 

Question 2. Pick out the concave and convex mirrors from the following and tabulate them. Rear-view mirror, Dentist's mirror, Torch-light mirror, Mirrors in shopping malls, Make-up mirror.
Answer: Here is the classification of the given mirrors:

In simple words: Concave mirrors are for seeing things bigger or for making strong, focused light beams. Convex mirrors are for seeing a wider area, making things look smaller but covering more space.

🎯 Exam Tip: Understand the primary function of each mirror type: concave for magnification and focusing, convex for wider field of view and diminshed images. This will help in classifying their applications.

 

Question 3. State the direction of the incident ray which after reflection from a spherical mirror retraces its path. Give a reason for your answer.
Answer: When an incident ray is directed towards the centre of curvature, it retraces its path after reflection. This is because at all the points of the spherical mirror, a ray directed towards the centre of curvature strikes the mirror surface normally (perpendicularly). Therefore, the angle of incidence \( i = 0^\circ \) and the angle of reflection \( r = 0^\circ \).
In simple words: A light ray that passes through the center point of a curved mirror will always bounce back exactly the way it came. This happens because it hits the mirror surface straight on, without any angle.

🎯 Exam Tip: This ray tracing rule is crucial for constructing accurate ray diagrams for spherical mirrors, especially when the object is placed at or beyond the centre of curvature.

 

Question 4. What is meant by magnification? Write its expression. What is its sign for real image and virtual image?
Answer: Magnification is the increase in size of an image compared to its true size. It is the ratio of the height of the image to the height of the object. It can also be expressed in terms of image and object distances.
The expression for magnification \( m \) is:
\[ m = \frac{\text{height of the image } h_2}{\text{height of the object } h_1} = -\frac{\text{image distance } v}{\text{object distance } u} \]
For a real image, the sign of magnification is negative (as real images are typically inverted).
For a virtual image, the sign of magnification is positive (as virtual images are typically erect).
In simple words: Magnification tells us how much bigger or smaller an image is compared to the actual object. If the image is upside down, the magnification number will have a minus sign; if it's upright, it will have a plus sign.

🎯 Exam Tip: The sign convention for magnification is critical: positive magnification indicates an erect (virtual) image, while negative magnification indicates an inverted (real) image. This helps describe image characteristics without seeing the diagram.

 

Question 5. Write the mirror formula and explain the meaning of each symbol used in it.
Answer: The expression relating the distance of the object \( u \), the distance of the image \( v \), and the focal length \( f \) of a spherical mirror is called the mirror formula or mirror equation. It is given as:
\[ \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \]
Here, the symbols represent:
\( f \) – focal length of the spherical mirror (distance from the pole to the principal focus).
\( u \) – distance of the object from the pole of the mirror.
\( v \) – distance of the image from the pole of the mirror.
All distances are measured from the pole of the mirror along the principal axis, and proper sign conventions must be applied.
In simple words: The mirror formula is a math rule that connects how far an object is from a mirror, how far its image is formed, and the mirror's focal length. Each letter in the formula stands for one of these distances.

🎯 Exam Tip: Always use the Cartesian sign convention consistently when applying the mirror formula to avoid errors in numerical problems. This means light travels from left to right, and distances measured against this direction are negative.

 

VIII. Answer in Detail:

 

Question 1. a) Draw ray diagrams to show how the image is formed using a concave mirror, when the position of object is (i) at C (ii) between C and F (iii) between F and P of the mirror. b) Mention the position and nature of image in each case.
Answer:
a) The ray diagrams for image formation by a concave mirror at different object positions are shown below:
(i) Object at C (Centre of Curvature)

Position of the object: At C
Position of the image: At C
Nature of the image:
(i) Real
(ii) Inverted
(iii) Same size as the object

(ii) Object between C (Centre of Curvature) and F (Focus)

Position of the object: Between C and F
Position of the image: Beyond C
Nature of the image:
(i) Real
(ii) Inverted
(iii) Magnified

(iii) Object between F (Focus) and P (Pole) of the Mirror

Position of object: Between F and P
Position of the image: Behind the Mirror
Nature of the image:
(i) Virtual
(ii) Erect
(iii) Magnified
In simple words: Concave mirrors form different types of images depending on where you place the object. When the object is at C, the image is real and the same size. When it's between C and F, the image gets bigger and is still real. But if it's very close, between F and P, the mirror acts like a magnifying glass, showing a big, upright image behind itself.

🎯 Exam Tip: Mastering these three specific ray diagrams for concave mirrors is essential, as they cover the most common object positions and image characteristics. Pay attention to the position of the image (real/virtual), its orientation (erect/inverted), and its size (magnified/diminished/same size).

 

Question 2. Explain with diagrams how refraction of incident light takes place from a) rarer to denser medium b) denser to rarer medium c) normal to the surface separating the two media.
Answer: Refraction is the bending of light as it passes from one medium to another. This happens due to the change in the speed of light.
a) Rarer to denser medium:
When a ray of light travels from an optically rarer medium (like air) to an optically denser medium (like water), it slows down. This causes the ray to bend towards the normal (the imaginary line perpendicular to the surface).

b) Denser to rarer medium:
When a ray of light travels from an optically denser medium (like glass) to an optically rarer medium (like air), it speeds up. This causes the ray to bend away from the normal.

c) Normal to the surface separating the two media:
A ray of light incident normally (at \( 90^\circ \) to the surface, or \( 0^\circ \) to the normal) on a denser medium does not undergo any deviation. It passes straight through without bending, as shown below.

In simple words: Light changes direction when it moves from one material to another because its speed changes. If it goes from a "thinner" material to a "thicker" one, it bends inwards. If it goes from "thicker" to "thinner," it bends outwards. But if it hits the surface straight on, it doesn't bend at all, just keeps going straight.

🎯 Exam Tip: Always remember that refraction only occurs when light strikes the interface at an angle. When light is incident normally (perpendicularly) to the surface, it passes undeviated, although its speed changes.

 

IX. Numerical Problems:

 

Question 1. A concave mirror produces three times magnified real image of an object placed at 7 cm in front of it. Where is the image located?
Answer: We are given the following:
Magnification \( m = -3 \) (since the image is real, it is inverted, hence negative magnification)
Object distance \( u = -7 \) cm (object is placed in front of the mirror, so it's negative according to sign convention).
The magnification formula for a mirror is \( m = -\frac{v}{u} \)
Substitute the given values:
\( -3 = -\frac{v}{-7} \)
\( -3 = \frac{v}{7} \)
\( v = -3 \times 7 \)
\( v = -21 \) cm
The image will be formed at a distance of 21 cm in front of the concave mirror from its pole. The negative sign for \( v \) indicates that the image is formed on the same side as the object, which is consistent with a real image.
In simple words: The mirror makes the object look three times bigger and upside down. If the object is 7 cm away, the image will be formed 21 cm away from the mirror, on the same side as the object.

🎯 Exam Tip: Pay close attention to the sign conventions for magnification and distances. A real image always implies negative magnification, and distances measured against the direction of incident light are negative.

 

Question 2. Light enters from air into a glass plate having a refractive index of 1.5. What is the speed of light in glass?
Answer: We are given:
Refractive index of glass plate \( \mu = 1.5 \)
Speed of light in vacuum \( C = 3 \times 10^8 \text{ ms}^{-1} \)
We need to find the speed of light in glass \( V \).
The formula for refractive index is:
\( \mu = \frac{\text{speed of light in vacuum (C)}}{\text{speed of light in medium (V)}} \)
Substitute the values:
\( 1.5 = \frac{3 \times 10^8}{V} \)
Now, we solve for \( V \):
\( V = \frac{3 \times 10^8}{1.5} \)
\( V = 2 \times 10^8 \text{ ms}^{-1} \)
Therefore, the speed of light in glass is \( 2 \times 10^8 \text{ ms}^{-1} \). This shows that light slows down when it enters a denser medium like glass.
In simple words: We know how fast light travels in empty space and how much glass slows it down (refractive index). Using a simple formula, we can figure out that light travels at 200 million meters per second when it passes through glass, which is slower than in a vacuum.

🎯 Exam Tip: Remember the relationship \( \mu = C/V \). This formula allows you to calculate the speed of light in any medium if you know its refractive index and the speed of light in a vacuum. Ensure units are consistent.

 

Higher Order Thinking Skills

 

Question 1. M = 2.42 for diamond. What is the meaning of this statement in relation to the speed of light?
Answer: This statement means that light travels 2.42 times faster in a vacuum compared to how fast it travels through a diamond. A higher refractive index indicates a slower speed of light in that medium.
In simple words: Light moves much slower inside a diamond than it does in empty space. It slows down by more than two times.

🎯 Exam Tip: Remember that refractive index is a ratio, indicating how much light slows down in a material compared to a vacuum.

 

Question 2. A convex mirror is in water. What should be the change in its focal length?
Answer: There will be no change in the focal length of the convex mirror. The focal length of a spherical mirror depends only on its geometric shape and not on the medium in which it is placed.
In simple words: Putting a convex mirror in water does not change how far its focus point is. The mirror's shape decides its focus, not what surrounds it.

🎯 Exam Tip: Focal length of mirrors depends on their curvature, while focal length of lenses depends on their material and the surrounding medium.

 

Question 3. Are the laws of reflection true in the case of irregular reflection?
Answer: Yes, the laws of reflection are true even in the case of irregular (or diffuse) reflection. At each point where light hits an uneven surface, the local reflection still follows the two laws of reflection: the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane, and the angle of incidence equals the angle of reflection. The surface only appears irregular because the normals at different points are in different directions.
In simple words: Even on a rough surface, each tiny spot where light hits still reflects light perfectly following the rules. It looks messy because all those tiny spots face in different directions.

🎯 Exam Tip: Diffuse reflection allows us to see objects from all angles, as light is scattered in many directions, unlike specular reflection from smooth surfaces.

 

Intext Activities

 

Activity - 1

 

Question. Stand before the mirror in your dressing table or the mirror fixed in a steel almirah. Do you see your whole body? To see your entire body in a mirror, the mirror should be atleast half of your height. Height of the mirror= Your height/2. What do you observe?
Answer: If a person is 5 feet tall, they would need a plane mirror that is at least 2.5 feet high to see their full body. If the mirror is this height and positioned correctly, the person's entire body will be visible in the mirror. This is because the minimum height required for a plane mirror to show a person's full image is exactly half of their height.
In simple words: To see your whole body in a flat mirror, the mirror needs to be at least half your height. This helps you see your reflection from head to toe.

🎯 Exam Tip: This principle applies universally to plane mirrors, regardless of how far you stand from the mirror.

 

Activity - 2

 

Question. Hold a concave mirror in your hand (or place it in a stand). Direct its reflecting surface towards the sun. Direct the light reflected by the mirror onto a sheet of paper held not very far from the mirror. Move the sheet of paper back and forth gradually until you find a bright, sharp spot of light on the paper. Position the mirror and the paper at the same location for few moments. What do you observe? Why does the paper catch fire?
Answer: When a concave mirror is pointed at the sun, it gathers all the incoming parallel light rays and brings them together. These light rays converge and meet at a single bright, sharp spot called the principal focus of the mirror. If a piece of paper is placed exactly at this focal point for a few moments, it will heat up intensely and can catch fire. This happens because the concave mirror concentrates a large amount of solar energy (heat and light) onto a very small area, causing a rapid temperature increase.
In simple words: A concave mirror can collect sunlight and focus it into one very hot spot. If you put paper there, it gets so hot it can burn because all the sun's energy is squeezed into a tiny area.

🎯 Exam Tip: Concave mirrors are also known as converging mirrors because they bring parallel rays of light to a single focus. This property is used in solar concentrators.

 

Activity - 3

 

Question. Take a convex mirror. Hold it in one hand. Hold a pencil close to the mirror in the upright position in the other hand. Observe the image of the pencil in the mirror. Is the image erect or inverted? Is it diminished or enlarged? Move the pencil slowly away from the mirror. Does the image become smaller or larger? What do you observe?
Answer: When a pencil is held upright in front of a convex mirror, the image formed is virtual, erect (upright), and diminished (smaller) than the actual pencil. As the pencil is moved slowly away from the convex mirror, the image remains erect but continues to become smaller and shifts closer to the mirror's focus. This always happens with convex mirrors; they always form smaller, upright images, which is why they are useful as rearview mirrors in vehicles.
In simple words: With a convex mirror, a pencil will always look smaller and stand upright. Moving the pencil further away makes its image even smaller, but it still stays upright.

🎯 Exam Tip: Convex mirrors always form virtual, erect, and diminished images, regardless of the object's position, providing a wider field of view.

 

Activity - 4

 

Question. Put a straight pencil into a tank of water or beaker of water at an angle of 45° and look at it from one side and above. How does the pencil look now?
Answer: When a straight pencil is placed into water at an angle and viewed from above, the part of the pencil submerged in water will appear to be bent or broken at the water's surface. This visual distortion happens because light rays change their speed and direction when moving from one medium (air) to another (water), a phenomenon called refraction. The light from the submerged part of the pencil bends as it leaves the water and enters the air, causing our eyes to perceive the pencil in a different position.
In simple words: A pencil in water looks bent because light changes direction when it goes from water to air. This change makes our eyes see the pencil in a slightly different spot.

🎯 Exam Tip: Refraction is also responsible for making objects under water appear shallower than they actually are, due to the bending of light rays.

 

9th Science Guide Light Additional Important Questions And Answers

 

I. Choose The Correct Answer:

 

Question 1. A ray of light is incident towards a plane mirror at an angle of 30° with the mirror surface. What will be the angle of reflection?
(a) 45°
(b) 30°
(c) 90°
(d) 60°
Answer: (d) 60°
In simple words: If light hits a flat mirror at a 30° angle to the mirror itself, its angle from the normal (the line straight out from the mirror) is 60°. So, it will bounce off at the same 60° angle.

🎯 Exam Tip: Always remember that the angle of incidence and reflection are measured with respect to the normal (perpendicular) to the surface, not the surface itself.

 

Question 2. A 10 mm long bin is placed vertically in front of a concave mirror. A 5 mm long image of the bin is formed at 30 cm in front of the mirror. The focal length of this mirror is
(a) -20cm
(b) -30cm
(c) -60cm
(d) -40cm
Answer: (a) -20cm
In simple words: If an object is 10mm long and its image is 5mm long at 30cm from the mirror, the mirror's focal length is -20cm. This negative sign shows it's a concave mirror.

🎯 Exam Tip: For concave mirrors, focal length is always negative, and you can use the magnification formula (image height/object height = -image distance/object distance) along with the mirror formula to find the focal length.

 

Question 3. A ray of light as it travels from medium A to medium B refractive index of the medium B relative to medium A is (μΒ/ μΑ)
(a) \( \frac{\sqrt{3}}{\sqrt{2}} \)
(b) \( \frac{\sqrt{2}}{\sqrt{3}} \)
(c) \( \frac{1}{\sqrt{2}} \)
(d) \( \sqrt{2} \)
Answer: (b) \( \frac{\sqrt{2}}{\sqrt{3}} \)
In simple words: The refractive index tells us how much light bends when it goes from one material to another. This value compares how light bends in medium B compared to medium A.

🎯 Exam Tip: The refractive index is a ratio that shows how much light's speed changes as it moves from one medium to another. It's often related to the sine of the angles of incidence and refraction (Snell's Law).

 

Question 4. Under which of the following conditions a concave mirror can form an image larger than the actual object?
(a) when the object is kept at a distance equal to its radius of curvature.
(b) when object is kept at a distance less than its focal length.
(c) when object is placed between the focus and centre of curvature
(d) ept at a distance greater than its radius of curvature.
Answer: (c) when object is placed between the focus and centre of curvature
In simple words: A concave mirror makes things look bigger if the object is placed somewhere between its focus point and its center of curve. Placing the object at the focus creates an image at infinity.

🎯 Exam Tip: Remember the different positions of an object in front of a concave mirror and the corresponding image characteristics (real/virtual, inverted/erect, magnified/diminished).

 

Question 5. In torches, searchlights and head lights of vehicles the bulb is placed ......... of the concave mirror.
(a) between and F of the reflector
(b) Very near to F
(c) between F&C
(d) at C
Answer: (b) Very near to F
In simple words: In torches and car headlights, the light bulb is put very close to the mirror's focus. This helps the mirror send out a strong, straight beam of light.

🎯 Exam Tip: For a concave mirror, when an object is placed at the focus, the reflected rays travel parallel to the principal axis, creating a powerful beam of light.

 

Question 6. A boy is standing at a distance of 3m in front of a plane mirror. The distance between the boy and his image is .... m
(a) 4
(b) 2
(c) 3
(d) 3
Answer: (c) 3
In simple words: If a boy stands 3 meters in front of a flat mirror, his reflection will appear 3 meters behind the mirror. The total distance between the boy and his image, as perceived, is then 6 meters. However, the provided answer says 3m. This might refer to the distance from the mirror.

🎯 Exam Tip: For a plane mirror, the image is formed as far behind the mirror as the object is in front of it. So the distance between the object and image is twice the object distance.

 

Question 7. The image formed by a concave mirror is real, inverted and of the same size as that of the object the position of the object should be
(a) beyond C
(b) (c) at C
Answer: (c) at C
In simple words: When a concave mirror creates an image that is real, upside down, and the same size as the actual item, the item must be placed exactly at the center of curvature.

🎯 Exam Tip: Knowing the image characteristics for various object positions of concave mirrors is crucial for solving such problems.

 

Question 8. Which of the following has the highest refractive index
(a) air
(b) water
(c) diamond
(d) glass
Answer: (c) diamond
In simple words: Out of air, water, diamond, and glass, diamond slows down light the most, which means it has the highest refractive index. This property makes diamonds sparkle a lot.

🎯 Exam Tip: A higher refractive index means light bends more when entering the material and also travels slower within it, leading to phenomena like brilliance in diamonds.

 

Question 9. The image formed by a plane mirror is
(a) real
(b) diminished
(c) enlarged
(d) laterally inverted
Answer: (d) laterally inverted
In simple words: A flat mirror shows an image that is virtual, upright, the same size, and flipped left-to-right (laterally inverted). For example, your right hand appears as the left hand of your image.

🎯 Exam Tip: Understanding the properties of images formed by plane mirrors (virtual, erect, same size, laterally inverted, and same distance behind) is fundamental to optics.

 

Question 10. The incident ray passing through 'F of a mirror ......... after reflection
(a) passes through C
(b) passes through F
(c) passes parallel to the principal axis
(d) the pole
Answer: (c) passes parallel to the principal axis
In simple words: When a light ray goes through the focus point of a mirror and hits the mirror, it will bounce back moving straight and parallel to the main line of the mirror. This is a key rule for how mirrors reflect light.

🎯 Exam Tip: This is one of the four principal rays used for drawing ray diagrams of spherical mirrors. Knowing these rules helps determine image formation.

 

Question 11. The incident ray passing through C of a mirror ......... after reflection.
(a) passes through C
(b) passes through F
(c) passes through P
(d) parallel to the principal axis
Answer: (a) passes through C
In simple words: If a light ray goes through the center of curvature of a mirror and hits it, it will bounce back along the very same path. This happens because the ray hits the mirror at a 90-degree angle.

🎯 Exam Tip: Rays passing through the center of curvature strike the mirror normally (perpendicularly) and thus retrace their path after reflection.

 

Question 12. The incident ray parallel to the principal axis of a mirror ......... after reflection.
(a) passes through C
(b) passes through F
(c) passes through P
(d) reverts back in the opposite direction
Answer: (b) passes through F
In simple words: When a light ray hits a mirror while traveling parallel to its main axis, it will always bounce back through the mirror's focus point. This is how mirrors bring light together or spread it out.

🎯 Exam Tip: This is another fundamental rule for ray diagrams, especially for concave mirrors where parallel rays converge at the focus, and for convex mirrors where they appear to diverge from the focus.

 

Question 13. According to sign convention the distance of the object.
(a) is always positive
(b) is always negative
(c) maybe positive or negative
(d) t height.
Answer: (b) is always negative
In simple words: When we use the standard rules for measuring distances in optics, the distance of the object from the mirror is always considered negative. This is because objects are generally placed to the left of the mirror.

🎯 Exam Tip: In Cartesian sign convention, distances measured against the direction of incident light are taken as negative, and object distance is usually measured this way.

 

Question 14. According to sign convention the distance of the image.
(a) is always positive
(b) is always negative
(c) maybe positive or negative
(d) is equal to image height
Answer: (c) maybe positive or negative
In simple words: The distance of the image from the mirror can be positive or negative. It's positive if the image is virtual (behind the mirror) and negative if the image is real (in front of the mirror).

🎯 Exam Tip: Positive image distance indicates a virtual image (formed behind the mirror), while negative image distance indicates a real image (formed in front of the mirror).

 

Question 15. Total internal reflection will occur if the angle of reflection is
(a) 45°
(b) 60°
(c) 90°
(d) 99°
Answer: (c) 90°
In simple words: Total internal reflection happens when light tries to go from a dense material to a less dense one, and the angle it hits at is greater than a special "critical angle". When the angle of refraction is 90°, it means the light travels along the surface, which is the definition of the critical angle.

🎯 Exam Tip: Total internal reflection occurs when the angle of incidence in the denser medium exceeds the critical angle, causing all light to reflect back into the denser medium. The critical angle itself is defined as the angle of incidence for which the angle of refraction is 90°.

 

Question 16. Magnification for the............... image is always ...............
(a) real, positive
(b) real, negative
(c) virtual, negative
(d) virtual, positive
Answer: (b) real, negative
In simple words: When a mirror makes a real image, that image is always upside down. Because it's upside down, its magnification value is always negative.

🎯 Exam Tip: Magnification (m) is positive for erect (virtual) images and negative for inverted (real) images. Its absolute value indicates size change.

 

Question 17. If magnification is +1.5. The image is .................
(a) erect
(b) diminished
(c) real
(d) invected
Answer: (a) erect
In simple words: If the magnification number is positive, it means the image is upright, not upside down. A value like +1.5 also tells us it's larger than the original object.

🎯 Exam Tip: A positive sign for magnification always indicates an erect (upright) image, which is always virtual. A negative sign indicates an inverted (upside down) image, which is always real.

 

Question 18. The refractive index of a denser medium with respect to rarer medium is
(a) 1
(b) greater than 1
(c) less than 1
(d) negative
Answer: (b) greater than 1
In simple words: When you compare a denser material (like glass) to a lighter one (like air), the refractive index of the denser material will always be bigger than 1. This shows light slows down more in the denser material.

🎯 Exam Tip: The refractive index is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. Since light always travels slower in any medium than in a vacuum, the refractive index of any medium (except vacuum) will always be greater than 1.

 

Question 19. We can see objects because of
(a) reflection
(b) refraction
(c) transmission
(d) diffraction
Answer: (a) reflection
In simple words: We are able to see things around us because light bounces off them and travels to our eyes. This bouncing back of light is called reflection. Without it, objects would be invisible in illuminated spaces.

🎯 Exam Tip: While refraction, transmission, and diffraction are also light phenomena, reflection is the primary reason we perceive non-luminous objects.

 

Question 20. The image formed by a convex mirror is always
(a) real (b) enlarged
(c) virtual & enlarged
(d) diminished
Answer: (d) diminished
In simple words: A convex mirror always makes an image that looks smaller than the actual object. The image is also upright and appears behind the mirror.

🎯 Exam Tip: Convex mirrors are known for always producing virtual, erect, and diminished images, regardless of the object's position, which is why they offer a wide field of view.

 

Question 21. As you move an object always from a convex mirror, its image becomes............... and moves towards .................
(a) smaller, infinity
(b) smaller, focus
(c) enlarged, infinity
(d) enlarged, focus
Answer: (b) smaller, focus
In simple words: When an object moves away from a convex mirror, its image gets smaller and smaller. It also moves closer and closer to the mirror's focus point, but it never actually reaches it.

🎯 Exam Tip: For convex mirrors, as the object moves from infinity to the pole, the virtual image moves from the focus to the pole, always remaining diminished and erect.

 

Question 22. For a spherical mirror ................. is true.
(a) f = 2R
(b) R = 2f
(c) fR = 2
(d) fR = \( \frac { 1 }{ 2 } \)
Answer: (b) R = 2f
In simple words: For any round mirror, the distance to its center of curvature (R) is always exactly twice its focal length (f). So, if you know one, you can easily find the other.

🎯 Exam Tip: This relationship \( R = 2f \) or \( f = \frac{R}{2} \) is fundamental for spherical mirrors and is used in various calculations and ray diagrams.

 

Question 23. The mirror formula is...............
(a) \( \frac{1}{u} - \frac{1}{v} = \frac{1}{f} \)
(b) \( \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \)
(c) \( f = \frac{uv}{u+v} \)
(d) \( f = \frac{u+v}{uv} \)
Answer: (c) \( \frac{1}{f}=\frac{1}{u}+\frac{1}{v} \)
In simple words: The mirror formula is a math rule that connects the focal length (f) of a mirror, how far the object is (u), and how far the image is (v). It helps us figure out where images will form.

🎯 Exam Tip: Always use the Cartesian sign convention correctly when applying the mirror formula to avoid errors in calculations. This formula is critical for numerical problems.

 

Question 24. For a plane mirror, magnification m =
(a) 0
(b) 1
(c) ± 1
(d) ≤0
Answer: (b) 1
In simple words: For a flat mirror, the image is always the exact same size as the object. So, the magnification (how much bigger or smaller it looks) is always 1.

🎯 Exam Tip: Plane mirrors always produce images that are virtual, erect, and of the same size as the object, hence the magnification is +1.

 

Question 25. Magnification for convex mirror is
(a) always positive
(b) always negative
(c) some times positive
(d) 1
Answer: (a) always positive
In simple words: A convex mirror always creates images that are upright. Because the images are always upright, their magnification value is always positive.

🎯 Exam Tip: Positive magnification indicates an erect (upright) image, which is characteristic of all images formed by convex mirrors.

 

Question 26. If the angle of incidence i = 0, the angle of reflection r =
(a) 0°
(b)
(c) 180°
(d) 45°
Answer: (a) 0°
In simple words: When light hits a mirror straight on (meaning the angle of incidence is 0 degrees), it bounces straight back. So, the angle of reflection is also 0 degrees.

🎯 Exam Tip: This is a direct application of the law of reflection, which states that the angle of incidence equals the angle of reflection.

 

Question 27. Refractive index of a medium is .................
(a) speed of light in air to speed of light in vacuum
(b) speed of light in vacuum to speed of light in air
(c) focal length to object distance
(d) speed of light in the medium × speed of light in the air
Answer: (a) speed of light in air to speed of light in vacuum
In simple words: The refractive index of a material shows how much slower light travels in that material compared to how fast it moves in the air or vacuum. It's a way to measure how much light bends.

🎯 Exam Tip: The absolute refractive index is correctly defined as the ratio of the speed of light in vacuum (or air, approximately) to the speed of light in the given medium.

 

Question 28. Bending of light as it passes from one medium to another is called
(a) reflection
(b) diffraction
(c) refraction
(d) deviation
Answer: (c) refraction
In simple words: When light moves from one material (like air) into another (like water), it changes speed and direction. This change in direction, or bending, is called refraction.

🎯 Exam Tip: Refraction is a key phenomenon that explains how lenses work, how rainbows form, and why objects underwater appear distorted.

 

Question 29. Ratio of sine of angle of incidence to sine of angle of refraction is .................
(a) gravitational law
(b) law of reflection
(c) (d) snell's law.
Answer: (d) snell's law
In simple words: The rule that says the ratio of the sine of the angle of light hitting a surface to the sine of the angle of light bending away is always constant is known as Snell's Law. This constant is the refractive index.

🎯 Exam Tip: Snell's Law \( (\frac{\sin i}{\sin r} = \text{constant} = n) \) is fundamental to understanding how light refracts at the boundary between two different media.

 

Question 33. A bundle of glass threads, each of which is capable of transmitting messages using light waves is called
(a) microscope
(b) convex
(c) periscope
(d) optic fibre
Answer: (d) optic fibre
In simple words: Optic fibres are tiny glass threads that can carry information using light, like a super-fast highway for light signals. They allow for very efficient long-distance communication.

🎯 Exam Tip: Remember that optic fibres work based on total internal reflection, allowing light to travel long distances without much loss.

 

Question 34. A ray of light travelling in medium 1 strikes and travels into another transparent medium 2. If the speed of light is greater in medium 1, the ray will
(a) refract towards the normal
(b) have an angle of incidence smaller than be angle of refraction
(c) refract away from the normal
(d) undergo total internal reflection
Answer: (a) refract towards the normal
In simple words: When light moves from a fast medium to a slower medium, it bends closer to the normal line. This is because the speed change causes the light wave to change direction.

🎯 Exam Tip: Recall Snell's Law and how the change in speed of light between two media dictates the bending direction relative to the normal.

 

Question 35. A ray of light travels from air into a glass block as shown. It makes an angle of 30° with the surface of the block. If the refractive index of the glass is 1.5, what will be the angle of refraction?
(a) 35.26°
(b) 1.30°
(c) 48.59°
(d) 19.47°
Answer: (d) 19.47°
In simple words: When light goes from air to glass, it bends. The angle it bends at can be figured out using the refractive index of the glass, which tells us how much the light slows down.

🎯 Exam Tip: Always remember that the angle of incidence is measured with respect to the normal, not the surface. If given the angle with the surface, subtract it from 90° to get the angle of incidence.

 

Question 36. The field of view is maximum for .................... (* FOV is the extent of the observable area that is seen at any given instant)
(a) plane mirror
(b) concave mirror
(c) convex mirror
Answer: (c) convex mirror
In simple words: Convex mirrors make things look smaller, but they let you see a wider area around you. This is why they are used as side mirrors in cars.

🎯 Exam Tip: Convex mirrors always form virtual, erect, and diminished images, which is key to their wide field of view application.

 

Question 37. A real and enlarged image can be obtained by using a
(a) convex mirror
(b) plane mirror
(c) concave mirror
Answer: (c) concave mirror
In simple words: Only a concave mirror can make an image that is both real (can be caught on a screen) and bigger than the actual object, depending on where the object is placed.

🎯 Exam Tip: Concave mirrors are versatile; they can form real, inverted, magnified, diminished, or same-sized images, as well as virtual, erect, and magnified images. Remember the different positions of the object and the resulting image characteristics.

 

Question 38. Which of the following statements about total internal reflection is true?
(a) angle of incidence should be greater than the critical angle
(b) light must travel from a medium of higher refractive index to a medium of lower refractive index
(c) both (a) and (b)
Answer: (c) both (a) and (b)
In simple words: For light to bounce back completely inside a denser material, two things must happen: it must be moving into a less dense material, and it must hit the boundary at a very large angle.

🎯 Exam Tip: Total internal reflection is a key principle in technologies like optical fibers and endoscopes, making it an important concept to understand thoroughly.

 

Question 39. The focal length of a concave mirror is 5cm. Its radius of curvature is
(a) 5 cm
(b) 10 cm
(c) 2.5 cm
Answer: (b) 10 cm
In simple words: The radius of curvature of a mirror is simply twice its focal length. This relationship helps determine the mirror's shape and how it focuses light.

🎯 Exam Tip: Always remember the relationship \( R = 2f \) for spherical mirrors, where R is the radius of curvature and f is the focal length.

 

II. Fill in the blanks:

 

1. The ratio of the sine of the angle of incidence to the sine of ....................is constant.
Answer: angle of refraction
In simple words: When light bends as it passes from one material to another, the ratio between the sines of the incoming and outgoing angles is always the same. This constant is called the refractive index.

🎯 Exam Tip: This statement describes Snell's Law, a fundamental principle of refraction, so ensure you remember the exact terms.

 

2. A spherical mirror whose reflecting surface is curved outwards is called .................... mirror.
Answer: convex
In simple words: A mirror that bulges outwards, like the back of a spoon, is called a convex mirror. It spreads light out.

🎯 Exam Tip: Distinguish between concave (curved inwards, converging light) and convex (curved outwards, diverging light) mirrors by their reflecting surfaces.

 

3. All distances parallel to the principal axis are measured from the .................... of the mirror.
Answer: pole
In simple words: When measuring distances for mirrors, we always start from the exact center point on the mirror's surface, which is called the pole. This ensures consistent measurements.

🎯 Exam Tip: Understanding the reference point (pole) for measurements is crucial for correctly applying mirror formulas and sign conventions.

 

4. A negative sign in the value of magnification indicates that the image is ....................
Answer: real
In simple words: If the magnification number has a minus sign, it means the image is real and inverted. Real images are formed when light rays actually meet.

🎯 Exam Tip: A negative magnification value indicates a real and inverted image, while a positive value indicates a virtual and erect image.

 

5. Light is refracted or bent while going from one medium to another because of its .................... changes.
Answer: speed
In simple words: Light bends when it moves from one material to another because its speed changes. This change in speed causes the light ray to alter its path.

🎯 Exam Tip: Refraction is fundamentally caused by the change in the speed of light as it transitions between media with different optical densities.

 

III. State whether true or false. If false, correct the statement:

 

1. The critical angle is defined as the angle of incidence at which the total internal reflection starts to occur.
Answer: True.
In simple words: The critical angle is the special angle of incoming light where, if the angle is larger, the light will totally reflect inside the material.

🎯 Exam Tip: For total internal reflection to occur, the angle of incidence must be greater than the critical angle, and light must travel from a denser to a rarer medium.

 

2. The angle of incidence is equal to the angle of reflection for perfect reflection.
Answer: True.
In simple words: When light bounces off a smooth surface, the angle at which it hits is always the same as the angle at which it leaves.

🎯 Exam Tip: This is the second law of reflection and is fundamental to understanding how mirrors work.

 

3. The image formed in a plane mirror is always inverted.
Answer: False.
Correct statement: The image formed in a plane mirror is always erect.
In simple words: A plane mirror shows you an image that is upright, not upside down. What looks reversed is left and right, not up and down.

🎯 Exam Tip: Remember that plane mirrors produce virtual, erect, and laterally inverted images of the same size as the object, at the same distance behind the mirror.

 

4. A star appears twinkling into the sky because of the reflection of light by the atmosphere
Answer: False.
Correct statement: A star appears twinkling into the sky because of the refraction of light by the atmosphere.
In simple words: Stars seem to twinkle because their light gets bent many times by the moving air layers in our atmosphere before reaching our eyes. This bending changes how bright and where the star seems to be.

🎯 Exam Tip: Twinkling of stars is a phenomenon caused by atmospheric refraction, specifically the varying refractive index of air layers due to changes in temperature and density.

 

5. Mirage is an example of refraction and total internal reflection of light.
Answer: True.
In simple words: A mirage happens because light bends a lot in hot air and then totally reflects, making you see something that isn't really there, like water on a hot road.

🎯 Exam Tip: Mirages occur when light rays from distant objects are refracted through layers of air with different temperatures and thus different refractive indices, leading to total internal reflection.

 

6. Optical Fibres are based on the phenomenon of dispersion
Answer: False.
Correct statement: Optical Fibres are based on total internal reflection.
In simple words: Optical fibres work by trapping light inside them, making it bounce perfectly off the inner walls over and over again. This process is called total internal reflection, not dispersion.

🎯 Exam Tip: Dispersion refers to the splitting of white light into its constituent colors, while optical fibers primarily utilize total internal reflection for efficient light transmission.

 

7. A water tank appears shallower when it is viewed from the top due to refraction.
Answer: True.
In simple words: When you look at water, things at the bottom seem closer to the surface than they actually are because of how light bends when it leaves the water and enters your eyes.

🎯 Exam Tip: This apparent change in depth is due to the phenomenon of refraction, where light rays bend as they pass from water (denser medium) to air (rarer medium).

 

8. Twinkling of stars and Mirage are the two phenomena occurring due to refraction.
Answer: True.
Twinkling of stars and Mirage.
In simple words: Both the twinkling of distant stars and the illusion of water on a hot road are caused by light bending as it passes through different air layers.

🎯 Exam Tip: While both phenomena involve refraction, mirages also critically depend on total internal reflection at specific angles.

 

9. Angle of incidence is zero if a ray of light is incident normal to the surface separating the two media.
Answer: True.
In simple words: If a light ray hits a surface straight on, without any slant, its angle of incidence is zero. This means it doesn't bend when it passes through.

🎯 Exam Tip: When light strikes a surface perpendicularly (normal incidence), it passes through without deviation, meaning both the angle of incidence and refraction are 0°.

 

10. A real image is inverted and can be caught on the screen.
Answer: True.
In simple words: An image that is truly formed by light rays coming together is called a real image. It is always upside down and can be shown on a screen.

🎯 Exam Tip: Real images are formed by actual intersection of reflected/refracted rays, always inverted, and can be projected onto a screen. Virtual images cannot.

 

11. The minimum length of the mirror required to see the full image of the person is half ' of his height.
Answer: true.
In simple words: To see your entire self in a mirror, the mirror only needs to be half as tall as you are. This is a neat trick of physics.

🎯 Exam Tip: This principle applies to plane mirrors; the minimum vertical length of the mirror required is half the height of the person, regardless of the person's distance from the mirror.

 

12. The pencil appears to be bent at the surface of the water is due to refraction.
Answer: True.
In simple words: When you put a pencil in water, it looks bent because light changes direction as it goes from water to air. This bending of light is called refraction.

🎯 Exam Tip: Refraction causes objects submerged in water to appear displaced or bent due to the change in optical density between water and air.

 

13. The speed of light decreases in a denser medium, light bends towards the normal.
Answer: True.
In simple words: When light enters a material that is more dense, it slows down. Because of this slowdown, the light ray bends closer to an imaginary line called the normal.

🎯 Exam Tip: This is a key rule of refraction: light bends towards the normal when moving from a rarer (faster) to a denser (slower) medium.

 

14. If the object is at infinity in front of a convex mirror the image is formed at infinity.
Answer: False.
Correct statement: The image is formed at F, behind the mirror
In simple words: For a convex mirror, if an object is very far away, the image will form at the mirror's focus, behind the mirror, and it will be tiny.

🎯 Exam Tip: For a convex mirror, objects at infinity always form a virtual, erect, and highly diminished image at the principal focus (F) behind the mirror.

 

15. An object is placed at a distance of 3cm from a plane mirror. The distance of the image is 3cm.
Answer: True.
In simple words: In a flat mirror, your reflection always appears to be the same distance behind the mirror as you are in front of it.

🎯 Exam Tip: A property of plane mirrors is that the object distance equals the image distance, and the image is formed behind the mirror.

 

16. The distance from centre of curvature of the mirror to the pole is called the focal length of the mirror.
Answer: False.
Correct statement: The distance between the centre of the mirror and the focal point of the mirror is called the focal length of a mirror.
In simple words: The distance from the center of curvature to the pole is actually the radius of curvature, not the focal length. The focal length is half of this distance.

🎯 Exam Tip: Clearly differentiate between focal length (f), which is half the radius of curvature (R), and the radius of curvature itself: \( f = R/2 \).

 

17. Light is one of the slowest travelling energy with a speed of \( 3 \times 10^{-8} \text{ms}^{-1} \)
Answer: False.
Correct statement: (Light is one of the fastest travelling energy with a speed of \( 3 \times 10^{8} \text{ms}^{-1} \)
In simple words: Light is incredibly fast, moving at about 300,000 kilometers per second in a vacuum. The number given in the question is incorrect by a very large margin.

🎯 Exam Tip: The speed of light in vacuum is approximately \( 3 \times 10^8 \text{ m/s} \) (or 300,000 km/s), which is the fastest speed known in the universe.

 

18. The angle of incidence at which the angle of refraction is Qf is called the critical angle.
Answer: False.
Correct statement: The angle of incidence at which the angle of refraction is 90° is called the critical angle.
In simple words: The critical angle is when light bends so much that it travels along the boundary between two materials, making an angle of 90 degrees with the normal.

🎯 Exam Tip: The critical angle is a specific angle of incidence where the refracted ray travels along the interface, leading to total internal reflection if the angle is exceeded.

 

IV. Match the following :

 

Question 1.

Answer:In simple words: Plane mirrors show images that are the same size and upright. Concave mirrors help dentists by making teeth look bigger. Convex mirrors allow you to see a very wide area.

🎯 Exam Tip: Understand the unique properties and practical applications of each type of mirror. This makes it easier to remember their characteristics.

 

Question 2.

Answer:In simple words: When the refracted angle is more than 90 degrees, it means the light is bouncing back inside (total internal reflection). If it's exactly 90 degrees, the light travels along the surface. If it's less than 90, the light has simply bent away from the normal.

🎯 Exam Tip: Relate the angle of refraction (r) to the critical angle and total internal reflection. When \( r = 90^\circ \), the angle of incidence is the critical angle. When \( r > 90^\circ \), total internal reflection occurs.

 

Question 3.

Answer:In simple words: Plane mirrors don't bend light to a single point, so their focal length is endless. Concave mirrors curve inwards and have a focal point in front, so their focal length is negative. Convex mirrors curve outwards and their focal point is behind them, making their focal length positive.

🎯 Exam Tip: Remember the sign conventions for focal length: concave mirrors (and converging lenses) have negative focal length, while convex mirrors (and diverging lenses) have positive focal length.

 

Question 4.

Answer:In simple words: If the magnification number is negative, it means the image is real and usually upside down. If the magnification number is positive, the image is virtual and upright.

🎯 Exam Tip: Remember the key connection: Negative magnification implies a real and inverted image, while positive magnification implies a virtual and erect image.

 

V. Assertion & Reason Type :
Mark the correct choice as:
(a) If both assertion and reason are true and reason is the correct explanation.
(b) If both assertion and reason are true and reason is not the correct explanation.
(c) If assertion is true but reason is false.
(d) If assertion & reason both are false.
(e) If assertion is false but reason is true.

 

Question 1. Assertion: The air bubble shines in water. Reason: Air bubble shines due to refraction of light.
Answer: (c) If assertion is true but reason is false
In simple words: Air bubbles in water look shiny because of total internal reflection, not just simple bending of light. The light bounces off the air-water surface from inside the water.

🎯 Exam Tip: The brilliance of air bubbles in water is a classic example of total internal reflection, where light rays from the denser medium (water) strike the interface with the rarer medium (air) at an angle greater than the critical angle.

 

Question 2. Assertion : The focal length of the mirror is /and distance of the object from the magnification of the mirror will be \( \frac{f}{f-u} \)
Reason : Magnification \( = \frac{\text{-image distance}}{\text{object distance}} = \frac{v}{u} \)
Answer: (a) Both assertion & reason are true and the reason is the correct explanation of the assertion
In simple words: The focal length and object distance help calculate how much an image is magnified. The magnification value itself is found by dividing the image distance by the object distance, but with a minus sign.

🎯 Exam Tip: Understand the mirror formula (\( \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \)) and the magnification formula (\( m = -\frac{v}{u} = \frac{h_i}{h_o} \)). The assertion's magnification formula is a derived form that combines these, useful for problem-solving.

 

Question 3. Assertion : When an object is placed between two plane mirrors, then all the images found are of equal brightness. Reason: Only two images are obtained in case of plane-parallel mirrors.
Answer: (d) Assertion & reason both are false
In simple words: When two flat mirrors face each other, you see many images, not just two, and they don't all look equally bright. The ones further away appear dimmer.

🎯 Exam Tip: When objects are placed between two parallel plane mirrors, an infinite number of images are formed, with each successive image becoming fainter due to loss of light at each reflection.

 

Question 4. Assertion: The mirrors used in torch lights are parabolic not concave. Reason: The image formed by concave mirror is always virtual.
Answer: (c) Assertion is true but reason is false
In simple words: Torches use special curved mirrors called parabolic mirrors, not standard concave ones, to create strong, focused light beams. The reason given is wrong because concave mirrors can make real images too, not just virtual ones.

🎯 Exam Tip: Parabolic mirrors are used in torches and headlights because they can produce a perfectly parallel beam of light when the source is placed at their focus, which a spherical concave mirror cannot do perfectly (it suffers from spherical aberration).

 

Question 5. Assertion : The nature of the image depends on the size of the mirror. Reason: Small mirrors always form a virtual image.
Answer: (d) Assertion & reason both are false
In simple words: How an image looks (its nature) depends on the mirror's curve and where the object is, not just its size. Also, small mirrors don't always make virtual images; it depends on their type (concave or convex).

🎯 Exam Tip: The nature of an image (real/virtual, erect/inverted) depends on the type of mirror (plane, concave, convex) and the object's position relative to the mirror's focal point and center of curvature, not merely the mirror's physical size.

 

Question 6. Assertion : A real image cannot be produced by plane or convex mirror. The focal length of a convex mirror is always taken as positive.
Answer: (e) Assertion is false but reason is true
In simple words: The first part is wrong because plane and convex mirrors always make virtual images, not real ones. But the second part is correct: a convex mirror's focal length is always considered positive.

🎯 Exam Tip: Convex mirrors and plane mirrors always form virtual, erect images. Their focal lengths are positive for convex mirrors and infinite for plane mirrors. Only concave mirrors can form real images under certain conditions.

 

VI. Answer very briefly :

 

Question 1. Which is optically denser out of the two medium M₁& M2 having the refractive indices = 1.71 and 1.36 respectively?
Answer: M1
Reason: Optical density increases as the value of the refractive index increases.
In simple words: The material with a higher refractive index is optically denser. Here, M1 has a refractive index of 1.71, which is higher than M2's 1.36. This means light travels slower in M1.

🎯 Exam Tip: A medium with a higher refractive index is considered optically denser because light travels slower through it compared to a medium with a lower refractive index.

 

Question 2. Two medium with refractive index 1.31 & 1.50 is given. In which case (i) Bending of light is more and (ii) speed of light is more.
Answer:
(i) 1.50 - Bending is more
(ii) 1.31 – Speed is more
In simple words: Light bends more when it enters a material with a much higher refractive index. It travels faster in the material with a lower refractive index.

🎯 Exam Tip: The greater the difference in refractive indices between two media, the more light bends (refracts). Conversely, light travels faster in a medium with a lower refractive index.

 

Question 3. Under what circumstances there won't be any refraction of light when it enters from one medium to another?
Answer:

• When light incident at 90°, it will not bend.
• When light passes from denser medium to rarer medium and if it is incident at an angle greater than the critical angle, it will reflect but will not refract.

In simple words: Light will not bend if it hits a surface straight on (at 90 degrees). Also, if light tries to leave a dense material at too steep an angle, it won't bend out, but instead bounce back inside, which is total internal reflection.

🎯 Exam Tip: Remember these two specific conditions for no refraction: normal incidence and total internal reflection, where light is entirely reflected rather than passing into the second medium.

 

Question 4. A ray of light traveling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Answer: When a light ray travels from air into water at an angle, it bends towards the normal line. This happens because water is optically denser than air. Due to this, the speed of light decreases when it enters the water, causing it to bend. Light always slows down when moving into a denser medium, which changes its direction.
In simple words: Light bends towards the normal when going from air to water because water is thicker (denser) than air, making the light slow down and change its path.

🎯 Exam Tip: Remember that light bends towards the normal when moving from a rarer (less dense) medium to a denser medium, and away from the normal when moving from denser to rarer.

 

Question 5. List down the uses of concave mirror.
Answer: Concave mirrors have several uses due to their ability to focus light or produce magnified images:
• They are used as shaving mirrors to see a magnified view of the face.
• They work as reflectors in car headlights and torches to produce a powerful, parallel beam of light.
• Dentists use them to get large, clear images of patients' teeth during examination.
• These mirrors are also employed in solar furnaces to gather and focus sunlight, generating heat.
In simple words: Concave mirrors are good for magnifying things, like in shaving mirrors, or for focusing light, like in car headlights and solar heaters.

🎯 Exam Tip: Concave mirrors are also used in reflecting telescopes to collect light from distant objects.

 

Question 6. What are the characteristics of the image formed on a plane mirror?
Answer: An image formed by a plane mirror has distinct characteristics:
• The image always appears upright, not upside down.
• It is the same size as the object placed in front of the mirror.
• The image appears to be behind the mirror at the same distance as the object is in front of it.
• It is laterally inverted, meaning left and right are swapped. Plane mirrors are flat and produce images that are a perfect reflection of the object.
In simple words: A plane mirror shows you an image that is virtual, upright, the same size as you, and seems to be behind the mirror, but it flips left and right.

🎯 Exam Tip: Focus on remembering the "V-L-E-S-D" acronym: Virtual, Laterally inverted, Erect, Same size, Same distance.

 

Question 7. State the laws of reflection of light.
Answer: The reflection of light follows two main laws:
• The incident ray (incoming light), the reflected ray (bounced light), and the normal (an imaginary line perpendicular to the surface at the point of incidence) all lie in the same plane. This means they are all on a flat surface.
• The angle of incidence (angle between incident ray and normal) is always equal to the angle of reflection (angle between reflected ray and normal). This principle ensures that light bounces off surfaces in a predictable way.
In simple words: The first law says all the light rays involved in reflection stay on one flat surface. The second law says the angle at which light hits a mirror is always the same as the angle at which it bounces off.

🎯 Exam Tip: Always draw the normal at the point of incidence to correctly measure the angles of incidence and reflection.

 

Question 8. Describe the nature of images formed by plane mirrors.
Answer: Images formed by plane mirrors have these qualities:
• They are always virtual, meaning the light rays do not actually meet at the image location, and erect, meaning they are upright.
• The image is the same size as the object.
• It is formed at the same distance behind the mirror as the object is in front of it.
• The image is laterally inverted, which means the left and right sides of the object appear swapped in the image. Understanding these characteristics helps in daily applications of mirrors.
In simple words: A plane mirror image is not real, it stands up straight, is the same size as the actual thing, appears as far behind the mirror as the object is in front, and swaps left and right.

🎯 Exam Tip: Think about your reflection in a regular mirror – it's always virtual, upright, and appears to be behind the mirror.

 

Question 9. What is lateral inversion in a plane mirror?
Answer: Lateral inversion describes the phenomenon where the left and right sides of an object appear to be swapped in its mirror image. For example, if you raise your right hand, your reflection appears to raise its left hand. This effect is why ambulance signs are often written backward so they appear correctly when viewed in a rearview mirror.
In simple words: Lateral inversion is when a mirror flips things sideways, so your left hand looks like your right hand in the reflection.

🎯 Exam Tip: Lateral inversion only swaps left-right, not up-down. The image remains erect.

 

Question 10. Explain why a ray of light passing through the centre of curvature of a concave mirror, gets reflected along with the same pattern.
Answer: When a ray of light passes through the center of curvature (C) of a concave mirror and hits the mirror's surface, it strikes the mirror along its normal. The normal line at any point on a spherical mirror passes through the center of curvature. According to the law of reflection, if a ray hits a surface perpendicularly (i.e., along the normal), its angle of incidence is 0°. This means the angle of reflection must also be 0°, so the ray bounces back along the exact same path. This principle is key for understanding how concave mirrors form images.
In simple words: A light ray going through the center of a concave mirror hits the mirror straight on. Because it hits straight, it bounces right back the way it came.

🎯 Exam Tip: Rays passing through C are special because they define the perpendicular direction to the curved surface at that point.

 

Question 11. How tall does a mirror have to be to fit an entire person's body?
Answer: To see a full image of a person, the minimum height of a plane mirror required is exactly half the person's height. This rule applies no matter how far the person stands from the mirror. This happens because light rays from the top and bottom of the person's body only need to travel half the vertical distance to reach the eye after reflection.
In simple words: A mirror only needs to be half your height for you to see your whole body in it, no matter how close or far you stand.

🎯 Exam Tip: This principle applies only to plane mirrors and is a common conceptual question in optics.

 

Question 12. What is concave and convex mirror?
Answer: Mirrors are classified based on the curvature of their reflecting surface:
• A concave mirror has a reflecting surface that is curved inwards, like the inside of a spoon. It focuses light rays towards a single point.
• A convex mirror has a reflecting surface that is curved outwards, like the back of a spoon. It diverges light rays outwards. These different shapes cause light to reflect in distinct ways, leading to different types of images.
In simple words: A concave mirror bends inward and gathers light, while a convex mirror bends outward and spreads light.

🎯 Exam Tip: Remember that "concave" mirrors 'cave in', and "convex" mirrors curve outwards like a 'V' and an 'X' pushing out.

 

Question 13. Define principal focus of concave mirror.
Answer: The principal focus (F) of a concave mirror is a specific point on its principal axis. It is the point where all light rays that are parallel to the principal axis meet after they are reflected from the mirror. This focal point is real and is located in front of the concave mirror. This characteristic allows concave mirrors to concentrate light, which is useful in many applications.
In simple words: For a concave mirror, the principal focus is where all parallel light rays meet after hitting the mirror.

🎯 Exam Tip: For concave mirrors, the principal focus is always real and located in front of the mirror.

 

Question 14. What is focal length (f) of a mirror?
Answer: The focal length (f) of a spherical mirror is the distance between its pole (P) and its principal focus (F). The pole is the center of the mirror's reflecting surface. This distance determines how strongly the mirror converges or diverges light. For concave mirrors, the focal length is usually considered positive, and for convex mirrors, it's negative in some conventions. The focal length is a fundamental property of any mirror.
In simple words: Focal length is the distance from the center of a mirror to the point where light rays meet or seem to meet after bouncing off.

🎯 Exam Tip: Remember the relationship: Radius of curvature (R) is twice the focal length (f), so \( R = 2f \).

 

Question 15. Define Radius of curvature.
Answer: The radius of curvature (R) of a spherical mirror is the radius of the hollow sphere from which the mirror is a part. It represents the distance from the center of curvature (C) to any point on the mirror's surface. The center of curvature is the center of that imaginary sphere. A key relationship is that the radius of curvature is twice the focal length, i.e., \( R = 2 \times \text{focal length} \). This relationship is important for understanding mirror properties.
In simple words: The radius of curvature is the radius of the big imaginary ball that a curved mirror is cut from.

🎯 Exam Tip: The center of curvature (C) is where the "center" of the sphere is, from which the mirror is a small piece.

 

Question 16. What is "aperture"?
Answer: The aperture of a spherical mirror refers to the effective diameter of its circular reflecting surface. It is essentially the portion of the mirror that is available for light to strike and be reflected. A larger aperture allows more light to be collected, which is why telescopes often have large apertures to gather faint light from distant stars. It determines the light-gathering power of the mirror.
In simple words: The aperture of a mirror is simply the size of its opening, or how wide it is, which tells you how much light it can catch.

🎯 Exam Tip: Think of the aperture as the "window" through which light enters the mirror.

 

Question 17. Distinguish between real & virtual image.
Answer: Here's how real and virtual images differ:
**Real Images:**
1. Light rays actually meet at the image location after reflection or refraction.
2. They can be formed (or caught) on a screen.
3. They are typically inverted (upside down) with respect to the object.
**Virtual Images:**
1. Light rays do not actually meet; they only appear to diverge from the image location.
2. They cannot be formed on a screen.
3. They are always erect (upright) with respect to the object. Understanding this difference is crucial for studying optics. Real images are formed by actual intersection of light rays, while virtual images are formed by apparent intersection.
In simple words: Real images can be seen on a screen and are upside down, because light rays truly meet. Virtual images cannot be caught on a screen and stand upright, because light rays only seem to meet.

🎯 Exam Tip: A simple way to remember is that real images are formed when light rays actually converge, like in a camera projector, while virtual images are what you see in a plane mirror.

 

Question 18. What do you mean by linear magnification?
Answer: Linear magnification (\( m \)) is a measure that tells us how much larger or smaller an image is compared to the actual object. It is defined as the ratio of the height of the image (\( h_i \)) to the height of the object (\( h_o \)). It can also be expressed in terms of image distance (\( v \)) and object distance (\( u \)) as \( m = -\frac{v}{u} \). A positive magnification means the image is upright (erect), while a negative magnification means it is inverted. It helps to quantify the size change. \[ m = \frac{h_i}{h_o} \text{ (or) } m = -\frac{v}{u} \] Where:
\( h_i \) = height of the image
\( h_o \) = height of the object
\( v \) = image distance
\( u \) = object distance
In simple words: Linear magnification tells you if an image is bigger or smaller than the real object, and if it's upright or upside down. You find it by dividing the image height by the object height.

🎯 Exam Tip: Remember that a magnification value greater than 1 means the image is enlarged, less than 1 means diminished, and equal to 1 means same size.

 

Question 19. Which kind of mirrors are used in the shaving mirror? Why?
Answer: Concave mirrors are commonly used as shaving mirrors. They are chosen because they can produce a magnified, upright (erect), and virtual image. When a person holds the concave mirror close to their face, placing their face between the mirror's pole and its principal focus, the mirror creates an enlarged view, which makes it easier to shave precisely. The ability of concave mirrors to magnify helps in seeing small details clearly.
In simple words: Concave mirrors are used for shaving because they make your face look bigger and upright, which helps you see clearly.

🎯 Exam Tip: Always place the object (your face, in this case) between the pole and the principal focus to get an erect and magnified image from a concave mirror.

 

Question 20. Which mirror is used as a reflector? Why?
Answer: Concave mirrors are primarily used as reflectors in devices like torches, vehicle headlights, and searchlights. This is because a concave mirror can collect light from a small source placed at its focus and reflect it out as a powerful, parallel beam of light. This parallel beam travels a long distance and illuminates a wide area effectively. Their ability to converge light makes them ideal for these applications.
In simple words: Concave mirrors are used as reflectors in flashlights and car headlights because they can turn a small light into a strong, straight beam.

🎯 Exam Tip: To create a parallel beam, the light source must be placed exactly at the mirror's principal focus.

 

Question 21. Write the uses of concave mirror.
Answer: Concave mirrors are versatile and used in many applications:
1. As shaving mirrors: They provide a magnified view of the face.
2. As dentist's head mirrors: They help dentists focus light and see a small, enlarged area inside the mouth (like teeth or throat).
3. As reflectors: They are used in torches and vehicle headlights to produce a powerful, parallel beam of light.
4. In solar heaters: They focus sunlight to generate heat at a specific point for heating purposes. Their converging property is key to these applications.
In simple words: Concave mirrors are used in shaving, by dentists, in flashlights to make strong beams, and in solar heaters to focus sun's heat.

🎯 Exam Tip: Remember that the common theme for concave mirror uses is either magnification (shaving, dentist) or light convergence/focusing (reflectors, solar heaters).

 

Question 22. What do you observe when an object is placed anywhere between P and infinity in front of a convex mirror?
Answer: When an object is placed at any position between the pole (P) and infinity in front of a convex mirror, you will observe the following characteristics for the image:
• The image is always formed behind the mirror, specifically between the pole (P) and the principal focus (F).
• The image is always virtual (light rays only appear to diverge from it) and erect (upright).
• The image is always diminished, meaning it is smaller than the actual object. Convex mirrors always produce these types of images, which gives them a wide field of view.
In simple words: With a convex mirror, no matter where you put an object, the image will always be virtual, upright, smaller, and appear behind the mirror, between the center and the focal point.

🎯 Exam Tip: Convex mirrors are commonly used as rearview mirrors in vehicles because they provide a wider field of view by forming smaller images.

 

Question 23. What is the nature of the image formed by a concave mirror if the magnification produced by the mirror is +4?
Answer: If the magnification produced by a concave mirror is \( +4 \), the positive sign indicates that the image formed is virtual and erect (upright). The value of 4 (which is greater than 1) indicates that the image is enlarged, being four times the size of the object. This type of image is formed when the object is placed between the pole (P) and the principal focus (F) of the concave mirror. Knowing the sign and value of magnification helps determine the image properties.
In simple words: A magnification of \( +4 \) from a concave mirror means the image is virtual (not real), stands upright, and is four times bigger than the object.

🎯 Exam Tip: A positive magnification value always signifies a virtual and erect image, while a negative value signifies a real and inverted image.

 

Question 24. Between which two points of a concave mirror should an object be placed to obtain a magnification of -2?
Answer: To obtain a magnification of \( -2 \) from a concave mirror, the object should be placed between the principal focus (F) and the center of curvature (C). The negative sign in the magnification (\( -2 \)) indicates that the image formed is real and inverted. The value of 2 (greater than 1) signifies that the image is magnified, or enlarged. When the object is in this region, the concave mirror projects a magnified, real, and inverted image. This position is a common scenario in ray diagrams.
In simple words: For a concave mirror to make an image that is real, upside down, and twice as big (magnification of \( -2 \)), the object must be placed between its focal point (F) and its center of curvature (C).

🎯 Exam Tip: Real and inverted images are associated with negative magnification, and values greater than 1 (magnitude-wise) mean the image is enlarged.

 

Question 25. To obtain an image twice the size of the object, between which two points related to a concave mirror should an object be placed?
Answer: To obtain an image that is twice the size of the object using a concave mirror, the object should be placed between the principal focus (F) and the center of curvature (C). When the object is in this position, the concave mirror forms a real and inverted image that is magnified. This is a key configuration for understanding image formation in concave mirrors.
In simple words: To make an image twice as big with a concave mirror, you should put the object between its focal point and its center of curvature. The image will be real.

🎯 Exam Tip: Magnified real images are formed between F and C for concave mirrors, while virtual magnified images are formed between P and F.

 

Question 26. Draw a ray diagram and also state the position the relative size and nature of image formed by a concave mirror, when an object is placed at C.
Answer: When an object is placed at the center of curvature (C) of a concave mirror, the image formed has these properties:
Image Position: At C, itself
Size: Same size as the object
Nature: (i) Real (ii) Inverted. The ray diagram clearly shows how light rays converge at the center of curvature to form the image.In simple words: When an object is placed at the center of curvature of a concave mirror, the image forms right there too, it's the same size, real, and upside down.

🎯 Exam Tip: For objects placed at C, the image is formed at C, is real, inverted, and of the same size. This is a critical point to remember.

 

Question 27. Why a pencil partly immersed in water appears to be bent at the water surface.
Answer: A pencil partly immersed in water appears bent at the water surface due to the refraction of light. When light rays travel from water (a denser medium) to air (a rarer medium) and then enter our eyes, they bend. Our brain interprets these bent rays as if they traveled in straight lines, making the submerged part of the pencil appear to be at a different position, causing the illusion of bending. The extent of this bending depends on the refractive index of water compared to air. This is a common demonstration of how light changes direction.
In simple words: A pencil looks bent in water because light rays change direction when they move from water to air, making our eyes see the pencil in a different place.

🎯 Exam Tip: Remember that refraction occurs because light changes speed when it passes from one medium to another.

 

Question 28. How should a ray of light be incident on a rectangular glass slab so that it comes out from the opposite side of the slab without being displaced?
Answer: For a ray of light to pass through a rectangular glass slab without any lateral displacement, it should be incident normally on the surface. This means the ray hits the surface at an angle of incidence of \( i = 0^\circ \). When light strikes a surface perpendicularly, it passes straight through without bending, so it does not get displaced as it exits the other side. This is an important exception to light bending during refraction.
In simple words: To pass through a glass block without moving sideways, a light ray must hit the block straight on (at a 90-degree angle to the surface).

🎯 Exam Tip: When light strikes a surface at \( 90^\circ \) (normal incidence), both the angle of incidence and the angle of refraction are \( 0^\circ \), meaning no bending occurs.

 

Question 29. Why a convex mirror is preferred for rearview mirrors in cars?
Answer: Convex mirrors are chosen for rearview mirrors in cars for two primary reasons:
• They always form virtual, erect (upright), and diminished (smaller) images. This allows a larger area of the scene behind the vehicle to be visible within the mirror's limited size.
• They provide a wider field of view compared to plane mirrors or concave mirrors. This is crucial for drivers to monitor traffic and potential hazards over a broader area, enhancing safety. The smaller images compress a larger view into the mirror.
In simple words: Cars use convex mirrors for rearview because they show a wide area and make images look smaller, so you can see more behind you.

🎯 Exam Tip: Although convex mirrors offer a wider field of view, remember the warning: "Objects in mirror are closer than they appear" because the images are diminished.

 

Question 30. List four properties of the image formed by a convex mirror.
Answer: An image formed by a convex mirror always has these four properties:
• It is always formed behind the mirror, between the pole (P) and the principal focus (F).
• The image is always virtual (light rays don't actually meet) and erect (upright).
• Its size is always smaller than the object (diminished).
• The magnification for a convex mirror is always positive, indicating an erect image. These properties make convex mirrors useful for applications needing a wide field of view.
In simple words: Convex mirrors always make images that are behind the mirror, virtual, upright, smaller than the object, and have positive magnification.

🎯 Exam Tip: Convex mirrors only form one type of image, regardless of the object's position – always virtual, erect, and diminished.

 

Question 31. List four properties of the image formed by the concave mirror when the object is placed between F & P
Answer: When an object is placed between the principal focus (F) and the pole (P) of a concave mirror, the image formed has these specific properties:
• The image is formed behind the mirror.
• It is enlarged, meaning it is bigger than the actual object.
• The image is virtual, as the reflected light rays only appear to diverge from this location.
• It is erect (upright), not inverted. This specific object placement is used in applications like shaving mirrors to get a magnified view. The magnification will be positive.
In simple words: When an object is between the focal point and pole of a concave mirror, the image forms behind the mirror, looks bigger, is virtual, and stands upright.

🎯 Exam Tip: This is the only case where a concave mirror forms a virtual and erect image; for all other positions, it forms real and inverted images.

 

Question 32. What is meant by the refraction of light?
Answer: Refraction of light refers to the phenomenon where a light ray changes its direction or bends when it passes from one transparent medium to another. This change in direction occurs because light travels at different speeds in different media. For instance, light travels faster in air than in water, causing it to bend as it crosses the boundary between them. This bending is responsible for many optical illusions and is fundamental to how lenses work.
In simple words: Refraction is when light bends as it goes from one material to another, like from air to water, because its speed changes.

🎯 Exam Tip: The amount of bending (refraction) depends on the refractive index of the two media and the angle at which light strikes the surface.

 

Question 33. State the laws of refraction of light.
Answer: The laws of refraction of light, also known as Snell's laws, are:
(i) The incident ray (incoming light), the refracted ray (bent light), and the normal (imaginary line perpendicular to the surface) at the point of incidence, all lie in the same plane. This means they are all on a single flat surface.
(ii) The ratio of the sine of the angle of incidence (\( i \)) to the sine of the angle of refraction (\( r \)) is a constant for a given pair of media and for a light of a specific color. This constant is called the refractive index of the second medium with respect to the first medium. \[ \frac{\sin i}{\sin r} = \text{constant} \] This constant is called the refractive index of the second medium with respect to the first medium. These laws explain how light behaves when passing through different materials.
In simple words: The first law of refraction says that the incoming light, the bent light, and a straight line through them all stay on one flat surface. The second law says that for specific materials and light color, there's a fixed ratio between the angles of incoming and bent light.

🎯 Exam Tip: The constant in Snell's Law is the relative refractive index between the two media, which varies with the wavelength (color) of light.

 

Question 34. Define refractive index & write its unit.
Answer: The refractive index of a medium is a measure of how much the speed of light changes, and thus how much light bends, when it passes from one medium to another. Specifically, the refractive index of the second medium with respect to the first medium is defined as the ratio of the sine of the angle of incidence in the first medium to the sine of the angle of refraction in the second medium. The refractive index is a dimensionless quantity, meaning it has no unit, because it is a ratio of two similar quantities (angles or speeds). This makes it a pure number.
In simple words: Refractive index tells us how much light bends when it enters a material. It is a number without any unit.

🎯 Exam Tip: A higher refractive index means light bends more and travels slower in that medium.

 

Question 35. Define refractive index in terms of speed of light.
Answer: The refractive index of a medium can also be defined in terms of the speed of light. The absolute refractive index (\( \mu \)) of a medium is the ratio of the speed of light in vacuum or air (\( c \)) to the speed of light in that medium (\( v \)). \[ \mu = \frac{\text{speed of light in vacuum or air (c)}}{\text{speed of light in the medium (v)}} \] Similarly, the refractive index of medium 2 with respect to medium 1 (\( _1\mu_2 \)) is the ratio of the speed of light in medium 1 to the speed of light in medium 2. This definition highlights that light slows down as it enters denser media. \[ _1\mu_2 = \frac{\text{speed of light in medium 1}}{\text{speed of light in medium 2}} \]In simple words: Refractive index shows how much slower light travels in a material compared to how fast it travels in empty space or air.

🎯 Exam Tip: Since the speed of light in vacuum is the maximum speed, the absolute refractive index of any medium is always greater than or equal to 1.

 

Question 36. What is total internal reflection?
Answer: Total internal reflection (TIR) is a phenomenon where a ray of light is completely reflected back into the same denser medium from which it originated, instead of passing into a rarer medium. This occurs when two conditions are met: the light must travel from a denser medium to a rarer medium, and the angle of incidence must be greater than the critical angle. When these conditions are met, the angle of refraction would be greater than \( 90^\circ \), which is impossible, so the light reflects internally. This effect is seen in diamonds and optical fibers.
In simple words: Total internal reflection happens when light tries to leave a thick material for a thin one but bounces back completely, like a perfect mirror, because it hits at too sharp an angle.

🎯 Exam Tip: Remember the two key conditions for TIR: light must travel from denser to rarer medium, and the angle of incidence must exceed the critical angle.

 

Question 37. Define the critical angle.
Answer: The critical angle is a specific angle of incidence for which the angle of refraction becomes \( 90^\circ \). This phenomenon occurs when light travels from a denser medium to a rarer medium. If the angle of incidence increases beyond this critical angle, total internal reflection occurs, meaning the light ray will no longer pass into the rarer medium but will instead be completely reflected back into the denser medium. It is a threshold angle for TIR.
In simple words: The critical angle is the special angle at which light, when going from a thick material to a thin one, bends so much that it travels along the surface, making the angle of bending \( 90^\circ \).

🎯 Exam Tip: For angles of incidence smaller than the critical angle, refraction occurs, but for angles greater than the critical angle, total internal reflection occurs.

 

Question 38. What are the conditions to achieve total internal reflection?
Answer: To achieve total internal reflection, two specific conditions must be met:
• The light must travel from an optically denser medium to an optically rarer medium (e.g., from water to air, or from glass to air).
• The angle of incidence inside the denser medium must be greater than the critical angle. If either of these conditions is not satisfied, total internal reflection will not occur. These conditions ensure that light is completely reflected back into the denser medium.
In simple words: For total internal reflection to happen, light must go from a thicker material to a thinner material, and it must hit the surface at an angle bigger than a special 'critical' angle.

🎯 Exam Tip: If light travels from a rarer to a denser medium, total internal reflection can never occur.

 

Question 39. What is mirage? How it occurs?
Answer: A mirage is an optical illusion, often seen in deserts or on hot roads, where distant objects appear displaced or reflected, resembling pools of water. It occurs due to the refraction and total internal reflection of light from the sky by heated air near the ground.
Here's how it happens:
• The air directly above the ground becomes much hotter than the air higher up.
• Hot air is less dense and has a smaller refractive index than cooler, denser air.
• So, the optical density of air layers increases with height above the hot ground.
• Light rays from a distant object (like the sky) travel from cooler, denser air into progressively hotter, rarer air layers near the ground.
• As light moves from denser to rarer air, it bends away from the normal. When the angle of incidence exceeds the critical angle, total internal reflection occurs.
• These totally internally reflected rays from the sky appear to originate from the ground, creating the illusion of a water pool. This fascinating phenomenon is purely an effect of light bending through varying air densities.
In simple words: A mirage is when you see water where there isn't any, usually on hot ground. It happens because hot air near the ground bends light from the sky, making it look like a reflection from water.

🎯 Exam Tip: The key to a mirage is the gradient of refractive index in the air, caused by temperature differences, leading to total internal reflection.

 

Question 40. How do twinkling stars occur? (or) what is the cause of the twinkling of stars?
Answer: Stars appear to twinkle due to the phenomenon of atmospheric refraction. As light from distant stars travels through Earth's atmosphere, it passes through various layers of air with constantly changing densities, temperatures, and moisture levels. These atmospheric layers act like different optical media, causing the light rays to refract continuously and randomly. The light rays are bent by different amounts, making the apparent position of the star seem to shift slightly, and the intensity of light reaching our eyes fluctuates. This continuous change in the star's apparent position and brightness causes the twinkling effect. Planets do not twinkle because they are much closer and appear as larger sources of light, so the atmospheric effects average out.
In simple words: Stars twinkle because their light gets bent many times by the moving air in our atmosphere before it reaches our eyes, making them look like they're flickering.

🎯 Exam Tip: Remember that twinkling is an atmospheric effect, not a property of the star itself. It is caused by varying refractive indices in the atmosphere.

 

Question 41. What is the phenomenon used in optical fibre? Explain.
Answer: Optical fibers primarily use the phenomenon of total internal reflection for transmitting signals. Optical fibers are thin strands, typically made of high-quality composite glass or quartz, consisting of a central core surrounded by a cladding. The core has a higher refractive index than the cladding. When a light signal is directed into one end of the fiber at a suitable angle, it repeatedly undergoes total internal reflection off the core-cladding boundary. This allows the light to travel along the entire length of the fiber without significant loss, carrying audio, video, or data signals over long distances. This efficient transmission of light signals makes optical fibers indispensable for modern communication.
In simple words: Optical fibers work by using total internal reflection, where light bounces again and again inside the fiber without escaping, sending signals over long distances.

🎯 Exam Tip: For total internal reflection to occur in optical fibers, the core must have a higher refractive index than the cladding.

 

Question 42. Write any two uses of total internal reflection.
Answer: Total internal reflection has several important applications:
1. **Spectacular brilliance of diamonds:** Diamonds sparkle brilliantly due to total internal reflection. Their high refractive index and specific cut ensure that light entering them undergoes multiple internal reflections before exiting, creating a dazzling effect.
2. **Transmission of audio and video signals through optical fibers:** Optical fibers use total internal reflection to transmit information (audio, video, data) over long distances with minimal loss. This is the backbone of modern internet and telecommunication. Other uses include periscopes and medical endoscopes.
In simple words: Two uses of total internal reflection are making diamonds sparkle and sending internet signals through fiber optic cables.

🎯 Exam Tip: Total internal reflection is vital for many technologies and natural phenomena involving light guidance and brilliance.

 

Question 43. What are the examples of total internal reflection in nature?
Answer: Total internal reflection can be observed in various natural phenomena:
1. **Mirage:** This optical illusion, often seen on hot roads or in deserts, makes distant objects appear reflected as if in a pool of water. It is caused by the total internal reflection of light from the sky through layers of air with different temperatures and densities.
2. **Twinkling of stars:** While primarily due to refraction, the complex path of light through the atmosphere can involve total internal reflection under certain conditions, contributing to the perceived twinkling. Another example is the bright appearance of air bubbles in water.
In simple words: Mirages seen on hot roads and the sparkling of diamonds are natural examples of total internal reflection.

🎯 Exam Tip: Remember that the shimmering effect of air over a hot surface, which makes distant objects appear wavy, is also due to varying refractive indices causing reflection-like effects.

 

Question 44. Give two examples of the transparent medium that are denser than air.
Answer: Two common examples of transparent media that are optically denser than air are water and glass. Light travels slower in these media compared to air, causing it to bend towards the normal when passing from air into them. These materials are widely used in optics due to their clear properties and higher refractive indices. Another example would be diamond.
In simple words: Water and glass are two clear materials that are thicker than air, making light travel slower through them.

🎯 Exam Tip: Optically denser means light travels slower in that medium, not necessarily that its physical density (mass per volume) is higher.

 

Question 45. A coin in a glass beaker appears to rise as the beaker is slowly filled with water why?
Answer: A coin at the bottom of a glass beaker appears to rise or be at a shallower depth when the beaker is filled with water. This phenomenon is caused by the refraction of light at the air-water interface. Light rays from the coin travel from water (denser medium) to air (rarer medium) and bend away from the normal as they exit the water. Our eyes and brain trace these bent rays back in straight lines, making the coin appear to be higher than its actual position. This is an optical illusion resulting from light bending.
In simple words: A coin in a water-filled glass looks higher up because light from the coin bends when it leaves the water and enters the air, making our eyes think the coin is in a different spot.

🎯 Exam Tip: The apparent depth is always less than the real depth when viewing an object submerged in a denser medium from a rarer medium.

 

Question 46. Name the spherical mirror(s) that has/have
(i) Virtual principal focus
(ii) Real principal focus
Answer:
(i) **Convex** mirrors have a virtual principal focus. For a convex mirror, light rays parallel to the principal axis appear to diverge from a point behind the mirror after reflection, and this point is its virtual focus.
(ii) **Concave** mirrors have a real principal focus. For a concave mirror, light rays parallel to the principal axis actually converge and meet at a point in front of the mirror after reflection, and this point is its real focus. The type of focus (real or virtual) depends on how the mirror's surface curves.
In simple words: Convex mirrors have a virtual focus where light seems to come from, while concave mirrors have a real focus where light actually meets.

🎯 Exam Tip: Virtual focus is behind the mirror, while real focus is in front. This distinction is crucial for understanding mirror properties.

VII. Long Answers

 

Question 1. List the sign conventions for reflection of light by spherical mirrors.
(i) Write the formula for the spherical mirror.
(ii) Mirror Equation
Answer:
(i) **Sign Conventions for Spherical Mirrors (New Cartesian Sign Convention):**
• The object is always placed to the left side of the mirror. This ensures that the incident light rays always travel from left to right.
• All distances are measured from the pole (P) of the mirror. The pole is the central point of the mirror's reflecting surface.
• Distances measured in the direction of the incident light (rightward) are taken as positive. Distances measured opposite to the direction of incident light (leftward) are taken as negative.
• Heights measured perpendicular to and above the principal axis (upward) are considered positive.
• Heights measured perpendicular to and below the principal axis (downward) are considered negative.
These Cartesian sign conventions are essential for consistently applying mirror formulas and solving numerical problems.

(ii) **Mirror Equation and Linear Magnification:**
The mirror equation is a mathematical expression that relates the object distance (\( u \)), image distance (\( v \)), and focal length (\( f \)) of a spherical mirror. This fundamental equation is: \[ \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \] Where:
\( f \) = focal length of a spherical mirror
\( u \) = distance of the object from the mirror
\( v \) = distance of the image from the mirror
**Linear Magnification:**
Linear magnification (\( m \)) describes how much larger or smaller an image is relative to the object. It is defined as the ratio of the height of the image (\( h_i \)) to the height of the object (\( h_o \)). It can also be expressed in terms of object and image distances: \[ m = \frac{h_i}{h_o} \text{ (or) } m = -\frac{v}{u} \] Here:
\( h_i \) = height of the image
\( h_o \) = height of the object
The sign of magnification indicates the nature of the image (positive for erect, negative for inverted).
In simple words: Sign conventions are rules for using plus or minus signs with distances and heights in mirrors. The mirror formula links how far the object is, how far the image is, and the mirror's focal length. Magnification tells you if the image is bigger or smaller and if it's upright or upside down.

🎯 Exam Tip: Master the sign conventions thoroughly, as a single error in applying them can lead to an incorrect answer in numerical problems. Always draw a simple ray diagram to visualize the positions and nature of the image.

 

Question 2. State the type of mirror used as
(i) Convex mirror
(ii) Concave mirror
Answer:
(i) Convex mirror: This type of mirror gives a broad view, which means you can see a large area. It also makes images look upright and smaller than the real object. For example, when vehicles come closer to a driver using a convex mirror, their image looks bigger. When they move away, the image gets smaller.
(ii) Concave mirror: This mirror is used to see a larger version of a face or object. If the object is placed between the mirror's pole and its main focus point, the image formed will appear upright, enlarged, and virtual (meaning it cannot be caught on a screen). Concave mirrors are commonly used in shaving mirrors.
In simple words: Convex mirrors show a wide, smaller, upright view. Concave mirrors can show a larger, upright view when the object is close.

🎯 Exam Tip: Remember that convex mirrors are preferred for rearview applications because they offer a wider field of view, helping drivers see more of the road behind them.

 

Question 3. Write the rules for the construction of image by Concave mirrors, along with ray diagram.
Answer:
(i) Rule 1: A ray of light that passes through the center of curvature will reflect back along the very same path it came from.

(ii) Rule 2: A light ray that travels parallel to the mirror's main axis will pass through the focal point (focus) after it reflects off the mirror.

(iii) Rule 3: A light ray that passes through the focal point (focus) will reflect off the mirror and travel parallel to the main axis.

(iv) Rule 4: A ray of light that hits the mirror at its pole (P) will reflect back in such a way that the angle it makes with the principal axis before and after reflection is the same (angle of incidence equals angle of reflection).

In simple words: These rules show how light bounces off a curved mirror. Light going through the middle comes back the same way. Light hitting parallel bounces to the special point (focus). Light going through the special point bounces parallel. Light hitting the mirror's center bounces off at the same angle it came in.

🎯 Exam Tip: Always use a ruler and pencil for drawing ray diagrams to ensure accuracy in angles and positions, which are crucial for full marks.

 

Question 4. Describe the nature and location of the images for the different positions of object which is placed in front of the concave mirror.
Answer:

In simple words: The type of image you see in a concave mirror changes based on how far the object is from it. It can be small or big, upside down or upright, and sometimes it's like a real picture while other times it's a virtual one.

🎯 Exam Tip: When drawing ray diagrams, remember to use at least two principal rays to locate the image accurately. Label the object, image, focus (F), and center of curvature (C) clearly.

VIII. Higher-Order Thinking Skills

 

Question 1. M = 2.42 for diamond. What is the meaning of this statement in relation to the speed of light?
Answer: A refractive index of \( \mu = 2.42 \) for diamond means that light travels \( 2.42 \) times slower in diamond compared to how fast it travels in a vacuum. This high refractive index is what causes diamonds to sparkle so much. The speed of light changes when it moves from one material to another.
In simple words: If a diamond has a refractive index of 2.42, it means light moves 2.42 times slower in the diamond than it does in empty space.

🎯 Exam Tip: Remember that refractive index is always the ratio of the speed of light in vacuum to its speed in the medium (\( \mu = c/v \)). A higher refractive index means light travels slower in that material.

 

Question 2. A convex mirror is in water. What should be the change in its focal length?
Answer: The focal length of a convex mirror will not change if it is placed in water. This is because a mirror's focal length is determined by its curved shape, or its radius of curvature, and the material it is made from, not the medium around it. Lenses, however, have focal lengths that do change with the surrounding medium.
In simple words: The focal length of a convex mirror stays the same even in water because it depends on the mirror's shape, not what's around it.

🎯 Exam Tip: Distinguish between mirrors and lenses: mirror properties like focal length depend only on geometry, while lens properties are affected by the refractive index of the surrounding medium.

 

Question 3. Are the laws of reflection true in the case of irregular reflection?
Answer: Yes, the laws of reflection still apply even in the case of irregular or diffused reflection. At each tiny point where light hits an uneven surface, the local angle of incidence equals the angle of reflection. The light rays scatter in many directions because the surface is rough, causing each point to have a different normal line. This local adherence to the laws makes the overall reflection appear irregular.
In simple words: Yes, the rules of reflection still work for bumpy surfaces. Each tiny part of the bumpy surface reflects light correctly, but because the surface is uneven, the light scatters everywhere.

🎯 Exam Tip: Understand that diffused reflection makes surfaces visible from all angles, like a matte wall, whereas regular reflection (from smooth surfaces) only allows viewing from specific angles, like a mirror.