Selina Concise Solutions for ICSE Class 7 Physics Chapter 4 Light Energy

Points to Remember

1. Light is a form of energy which helps us to see objects.
2. Light always travels in a straight line in the form of rays.
3. Light sources are either natural or artificial.
4. The sun and stars are natural sources of light.
5. A bulb, a candle etc. are artificial sources of light.
6. The bodies which emit light themselves are called Luminous Bodies, e.g. sun, star.
7. The bodies which do not emit light are called non-luminous bodies, e.g. wood, brick etc.
8. Objects are of three types, transparent, translucent or opaque.
9. The pinhole camera is a simple application of the rectilinear propagation of light.
10. When an object blocks light, it casts a shadow.
11. Eclipses are formed due to formation of shadows.
12. Solar and lunar eclipses are the examples of formation of shadow in nature.
13. An eclipse is the partial or complete hiding of one heavenly body by shadow of another.
14. When light strikes a polished surface it comes back in the same medium, is called reflection of light.
15. A straight highly polished, smooth and reflecting surface is known as a plane mirror.
16. Mirrors are of two types
    (a) plane mirrors
    (b) spherical mirrors.
17. Smooth and polished surface like a mirror causes reflection and is called a regular reflection.
18. Rough or diffused surface causes an irregular reflection.
19. According to first law of reflection. The incident ray, the normal and the reflected ray all lie in the same plane.
20. According to the second law of reflection, the angle of incidence is always equal to the angle of reflection.
21. The phenomenon due to which the left side of an object appears to be the right side of the object and right side appears left. This is known as lateral inversion.
22. Image is of two types
    (a) Real image
    (b) Virtual image.

Test Yourself

A. Objective Questions

1. Write true or false for each statement

(a) The image formed by a plane mirror is real.
Answer: False. The image formed by a plane mirror is virtual.
A virtual image is one that cannot be obtained on a screen because the light rays do not actually meet there. It only appears to be behind the mirror when we look into it.
Teacher's Tip: Virtual starts with 'V' - think of a 'Vanishing' image that isn't really on a screen!
Exam Tip: Always write the corrected statement when an answer is False to show your full understanding.

(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the angle of reflection.
Answer: False. When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.
The laws of reflection apply to every surface, even rough ones like a wall. While the rays might scatter in different directions (irregular reflection), each individual ray still follows the law where i = r.
Teacher's Tip: Remember that the "Laws" of physics never take a break, no matter how rough the surface is!
Exam Tip: Mention "Irregular Reflection" if asked why we can't see our image in a wall despite the law holding true.

(c) The image of the right hand in a plane mirror looks like that of a left hand.
Answer: True.
This phenomenon is known as lateral inversion, where the sides of the object and image are swapped. It happens because the mirror reflects light directly back toward the source.
Teacher's Tip: Think of a handshake in the mirror; your right hand reaches out to what looks like the other person's left hand.
Exam Tip: Use the term "Lateral Inversion" whenever you describe side-swapping in mirrors.

(d) The image formed by a plane mirror is upright.
Answer: True.
An upright image means that the top of the object appears at the top of the image. This is also called an "erect" image, which is a standard characteristic of plane mirrors.
Teacher's Tip: Upright is the opposite of inverted (upside down).
Exam Tip: "Erect" and "Upright" mean the same thing in mirror physics; you can use either word.

(e) The image formed by a plane mirror can be obtained on a screen.
Answer: False. The image formed by a plane mirror cannot be obtained on a screen.
Images that cannot be caught on a screen are called virtual images. In a plane mirror, the light rays only seem to come from a point behind the glass.
Teacher's Tip: If you can't put a piece of paper where the image is and see it, it's virtual.
Exam Tip: Real images are formed by actual intersection of rays; virtual images are formed by produced rays.

(f) The objects are seen around us due to irregular reflection of light.
Answer: True.
Most surfaces around us are rough on a microscopic level, causing light to bounce off in many directions. This scattered light reaches our eyes from every angle, allowing us to see the object clearly.
Teacher's Tip: Without irregular reflection, everything would look like a confusing mirror!
Exam Tip: Note that irregular reflection is also called "diffused reflection."

(g) The speed of light in vacuum is 3 × 108 ms^-1.
Answer: True.
Light travels at this incredible speed, which is about 300,000 kilometers per second. It is the universal speed limit and is slightly slower when passing through air, water, or glass.
Teacher's Tip: Light can travel around the Earth 7.5 times in just one second!
Exam Tip: Always include the unit ms^-1 when writing the speed of light.

(h) A rose appears red in light of all the colours.
Answer: False. A rose appears red in white light.
An object only looks red because it reflects red light and absorbs other colors. If you shine only green light on a red rose, it will appear black because there is no red light to reflect.
Teacher's Tip: Objects don't "have" color; they "borrow" it from the light hitting them.
Exam Tip: Explain that color depends on the light reflected by the object back to our eyes.

(i) A black paper absorbs light of all the colours and reflects none.
Answer: True.
Black is the absence of reflected light, meaning the paper takes in all the energy from the light rays. This is why black clothes feel warmer in the sun than white ones.
Teacher's Tip: Black is like a sponge for light; it soaks it all up!
Exam Tip: Mention that total absorption of light results in the appearance of the color black.

(j) The primary colours are red, blue and green.
Answer: True.
These three colors of light can be mixed in different amounts to create every other color we see. When all three are mixed equally, they create white light.
Teacher's Tip: Use the acronym RGB (Red, Green, Blue) just like on computer screens.
Exam Tip: Do not confuse primary colors of light (RGB) with primary colors of paint (RYB).

2. Fill in the blanks

(a) Angle of incidence = angle of reflection
Answer: angle of reflection
This equality is the second law of reflection and it holds true for every smooth reflecting surface. If light hits a mirror at 45°, it must bounce away at 45°.
Teacher's Tip: Think of a bouncy ball hitting a flat floor; it comes up at the same angle it went down.
Exam Tip: In diagrams, ensure that the angle 'i' and angle 'r' look visually equal.

(b) The incident ray, the reflected ray and the normal lie in one plane
Answer: plane
This means that all three lines can be drawn on a single flat sheet of paper. They don't stick out in different directions into the air.
Teacher's Tip: 'In one plane' basically means they are all on the same flat surface.
Exam Tip: This is the first law of reflection; make sure to name the three lines involved.

(c) The image formed by a plane mirror is at a distance behind the mirror as the object is in front of it.
Answer: the object is in front of it.
In a plane mirror, the image distance always equals the object distance. if you stand 2 meters away from a mirror, your image looks like it is 2 meters deep inside.
Teacher's Tip: If you step back, your mirror twin steps back exactly the same amount!
Exam Tip: For numericals, remember that the total distance between the object and image is twice the distance to the mirror.

(d) The image formed by a plane mirror is erect and virtual.
Answer: virtual.
A plane mirror doesn't turn you upside down (erect) and you can't project that image onto a wall (virtual). These two properties are fixed for all plane mirrors.
Teacher's Tip: Erect = Heads up; Virtual = Can't touch it on a screen.
Exam Tip: "Erect and virtual" usually go together as a pair of characteristics for mirrors.

(e) We are able to see the objects around us due to irregular reflection.
Answer: irregular
Because surfaces are not perfectly smooth, they scatter light in every direction. This is what makes a book or a table visible from different sides of a room.
Teacher's Tip: Irregular reflection is the reason we can see things that aren't shiny mirrors.
Exam Tip: Distinguish between regular (mirror) and irregular (objects) reflection clearly.

(f) A virtual image cannot be obtained on a screen.
Answer: virtual
Virtual images are formed by rays that only appear to diverge from a point. Since the rays don't actually meet at that point, no energy is focused there to form a picture on a screen.
Teacher's Tip: Real images are 'Real' because they can really be put on a screen!
Exam Tip: Define a virtual image by its inability to be caught on a screen for full marks.

(g) One surface of mirror is made opaque by silvering it followed by a thin coating of paint of lead oxide. .
Answer: silvering
Silvering involves applying a thin layer of metal like silver or aluminum to the glass to make it reflect light. The lead oxide paint protects this delicate metal layer from scratches and air.
Teacher's Tip: The 'silver' part is what does the reflecting; the glass is just a holder.
Exam Tip: Know the materials used: silver for reflection and lead oxide for protection.

(h) A plane mirror does not reflect 100 percent light falling on it.
Answer: does not
No mirror is perfect; some light is always absorbed by the glass or the metal coating. High-quality mirrors reflect about 90-95% of light.
Teacher's Tip: In physics, nothing is ever 100% perfect due to tiny losses of energy.
Exam Tip: Remember that absorption always happens alongside reflection, even in small amounts.

(i) The colour of an opaque object is the colour of light which it reflects.
Answer: reflects.
If an object looks green, it is because it is rejecting (reflecting) green light back to your eyes while keeping all the other colors. Our eyes see what the object "throws away."
Teacher's Tip: Objects are color-thieves; they keep most colors but throw one back at you!
Exam Tip: Use the words "absorption" and "reflection" together to explain color.

(j) Magenta, cyan and yellow are the secondary colours.
Answer: secondary
Secondary colors are made by mixing two primary colors of light. For example, Red and Blue make Magenta.
Teacher's Tip: Secondary = Second level (made from a mix of level one primary colors).
Exam Tip: Be able to list which two primary colors make each secondary color.

3. Match the following

Column A
(a) A light ray passes from air to glass
(b) A light ray passes from glass to water
(c) Virtual image
(d) Red rose
(e) Red, green and blue
Column B
(i) speeds up
(ii) reflects red light
(iii) primary colours
(iv) plane mirror
(v) slows down

Column A - Column B
(a) A light ray passes from air to glass - (v) slows down
(b) A light ray passes from glass to water - (i) speeds up
(c) Virtual image - (iv) plane mirror
(d) Red rose - (ii) reflects red light
(e) Red, green and blue - (iii) primary colours
Answer: (a)-(v), (b)-(i), (c)-(iv), (d)-(ii), (e)-(iii)
Light speed changes depending on how thick (dense) the material is; it is fastest in air and slower in glass. This matching shows the relationship between light behavior and properties of objects.
Teacher's Tip: Thicker stuff (like glass) makes light 'swim' slower than thinner stuff (like air).
Exam Tip: When matching, look for obvious pairs like "Red Rose" and "reflects red light" first.

4. Select the correct alternative

(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 metre from himself. The distance of man from the mirror is
1. 6 m
2. 3 m
3. 2 m
4. 12 m
Answer: 2. 3 m
The total distance between the man and his image is 6 meters. Since the image is as far behind the mirror as the man is in front, we divide the total distance by 2 to get 3 meters.
Teacher's Tip: Total distance = (Man to Mirror) + (Mirror to Image). Since they are equal, it's just 3 + 3 = 6.
Exam Tip: Always divide the "Man-to-Image" distance by 2 to find the distance from the mirror.

(b) The angle between the incident ray and the ray reflected from the plane mirror is 70°. The angle of incidence will be :
1. 70°
2. 30°
3. 35°
4. 90°
Answer: 3. 35°
The angle between the two rays includes both the angle of incidence 'i' and the angle of reflection 'r'. Since i = r, we divide the total angle (70°) by 2 to find that i = 35°.
Teacher's Tip: The normal line cuts the total angle between rays perfectly in half!
Exam Tip: Read carefully—if the question says "angle between rays," you must divide by two to get 'i'.

(c) The image formed by a plane mirror is
1. virtual and inverted
2. virtual and of same size
3. real and inverted
4. real and of same size
Answer: 2. virtual and of same size
Plane mirrors always produce an image that is upright (not inverted) and exactly the same size as the object. It is virtual because it cannot be projected.
Teacher's Tip: You don't look bigger, smaller, or upside down in your bathroom mirror, right? That's your proof!
Exam Tip: "Virtual, erect, and same size" are the three key properties of plane mirror images.

(d) The angle of incidence on a plane mirror is 30°.The angle of reflection will be:
1. 30°
2. 60°
3. 15°
4. 0°
Answer: 1. 30°
According to the second law of reflection, the angle of incidence is always equal to the angle of reflection. If i = 30°, then r must also be 30°.
Teacher's Tip: This is the easiest question in physics—just copy the number!
Exam Tip: State the law "i = r" in your rough work to justify your choice.

(e) The angle of incidence on a plane mirror is 30°. The angle between the incident ray and the reflected ray is
1. 30°
2. 15°
3. 60°
4. 90°
Answer: 3. 60°
If the angle of incidence is 30°, then the angle of reflection is also 30°. The total angle between them is the sum of both (30° + 30° = 60°).
Teacher's Tip: "Between the rays" means i + r.
Exam Tip: Draw a small sketch to see that the two angles together make the total gap.

(f) The property due to which a light ray striking a surface is returned back into the same medium is called
1. refraction
2. reflex action
3. reflection
4. regression
Answer: 3. reflection
Reflection is the bouncing back of light when it hits a surface. Refraction is when light goes *through* and changes speed, but reflection stays in the same medium.
Teacher's Tip: Think of a mirror reflecting your face back into the air of the room.
Exam Tip: The keyword is "returned back into the same medium."

(g) A ray of light after reflection from a mirror is known as
1. reflected ray
2. normal
3. incident ray
4. refracted ray
Answer: 1. reflected ray
The ray that "hits" the mirror is the incident ray, and the one that "leaves" after bouncing is the reflected ray. The normal is just an imaginary line in between.
Teacher's Tip: Incident = Incoming; Reflected = Rebounding.
Exam Tip: Do not confuse reflected with refracted; refracted rays go *through* surfaces like water.

(h) The speed of light is maximum in
1. glass
2. water
3. air
4. wood
Answer: 3. air
Light travels fastest when there are fewer atoms in its way. Vacuum is the fastest, then air, then liquids like water, and then solids like glass.
Teacher's Tip: Light loves empty spaces; the less 'stuff' there is, the faster it goes!
Exam Tip: If "Vacuum" is an option, it is even faster than air.

(i) A red rose is seen in green light. It will appear.
1. red
2. blue
3. yellow
4. black
Answer: 4. black
A red rose can only reflect red light. If only green light is available, the rose absorbs it all and reflects nothing back to the eye, making it look black.
Teacher's Tip: Red + Green light (on a red object) = Darkness (Black).
Exam Tip: Objects appear black in a color of light that they cannot reflect.

(j) The primary colours are
1. Red, Blue and Yellow
2. Magenta, Yellow and Cyan
3. Red, Blue and Cyan
4. Blue, Green and Red
Answer: 4. Blue, Green and Red
These are the three colors that make up white light and cannot be made by mixing other light colors. All colors on your TV or phone screen are made using these three.
Teacher's Tip: Remember the lights on a remote control or power button are often Red or Green—these are primary light colors!
Exam Tip: Stick to Blue, Green, Red for light physics; Yellow is for art class.

B. Short/Long Answer Questions

Question 1: What do you mean by the term reflection of light ?
Answer: Reflection of light - When light strikes a polished surface it comes back in the same medium, is called reflection of light.
Just like a ball bouncing off a wall, light changes direction when it hits a shiny surface. This allows us to see our own image in mirrors and shiny metals.
Teacher's Tip: Think of reflection as a "light bounce."
Exam Tip: Mention "same medium" to distinguish reflection from refraction.

Question 2: How is a plane mirror made ?
Answer: To make a plane mirror, a thin piece of glass is taken. One surface of the mirror is made opaque by silvering it. On the top of that, another thin coating of red lead oxide is given which protects the silvering of the mirror.
The silvering creates a smooth metallic layer that reflects almost all light hitting it. The red paint is essential because it stops the metal from oxidizing or getting scratched.
Teacher's Tip: Glass is just the transparent 'bodyguard' for the thin reflective metal layer.
Exam Tip: List the two layers behind the glass: silvering and red lead oxide paint.

Question 3: Explain the following terms: Incident ray, Reflected ray, Angle of incidence, Angle of reflection, Normal.
Answer:
Incident ray - The ray of light falling on the surface AB is called the incident ray. In figure PN is the incident ray.
Reflected ray - The incident ray bouncing back in the same medium after striking the reflecting surface is called reflected ray. In figure NQ is the reflected ray.
Angle of incidence - The angle formed between the incident ray and the normal is the angle of incidence. (PNM is the angle of incidence.)
Angle of reflection - The angle formed between the normal and the reflected ray is called angle of reflection (MNQ is the angle of reflection)
Normal - It is the line drawn perpendicular to the reflecting surface at the point of incidence. MN is the normal.
These terms describe the geometry of how light interacts with a mirror. The Normal is an imaginary line that serves as the reference point for measuring all angles.
Teacher's Tip: "Normal" in physics means "perpendicular" (90° to the surface).
Exam Tip: Always measure angles from the Normal, not from the surface of the mirror.

Question 4: Draw a diagram showing the reflection of a light ray from a plane mirror. Label on it the incident ray, the reflected ray, the normal, the angle of incidence i and the angle of reflection r.
Answer: AO is the incident ray, OB is the reflected ray, ON is the normal, ∠ AON is the angle of incidence, ∠ NOB is the angle of reflection.
The diagram shows a flat line for the mirror with diagonal shading underneath to show the opaque side. A single line comes in (incident), a dashed vertical line represents the normal, and another line goes out (reflected).
Teacher's Tip: Use a ruler! Light travels in straight lines, so your diagram must have perfectly straight lines.
Exam Tip: Add arrows on the rays to show the direction of light flow (toward and away from the mirror).

Question 5: State the two laws of reflection of light.
Answer: Laws of reflection - (i) The incident ray, normal and the reflected ray all lie in the same plane. (ii) The angle of incidence is equal to the angle of reflection.
The first law ensures that the light doesn't "jump" out of its path into a third dimension. The second law (i=r) explains the predictable way light bounces off surfaces.
Teacher's Tip: Law 1 is about "where" they are; Law 2 is about the "angle."
Exam Tip: State both laws clearly using separate points to ensure full marks.

Question 6: Describe an experiment to verify the laws of reflection of light. Ans. Laws of reflection.
Answer: (i) The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane. (ii) The angle of incidence and angle of reflection are equal i.e. ∠ i = ∠ r.
Verification: Take a wooden drawing board and fix a white sheet of paper on it. In the middle of paper draw a straight line KK' Mark a point B on it. Draw a perpendicular BN. Place a mirror XX’ on line KK’ such that polished side of mirror is along the line. Hold the mirror in the mirror holder. Fix two steel pins P and Q on the straight line AB atleast 10 cm apart. Look for the images of the pins P and Q and fix two pins P' Q’ such that P’, Q’ and images of P and Q are all in the same straight line. Remove the pins and draw small circles around the pin pricks. Remove the mirror also. Join P’Q’ and produce the straight line to meet at B. Measure ∠ ABN = i and ∠ CBN = r. It is found that ∠ i = ∠ r. This proves that Angle of Incidence is equal to Angle of Reflection. As the incident ray, the reflected ray and the normal lie in the plane of paper, therefore, they lie in the same plane.
By aligning physical pins with their mirror reflections, we can map the exact path light takes. This hands-on method proves that light is predictable and follows geometry.
Teacher's Tip: The 'magic' happens when the pins and their images look like one single straight line!
Exam Tip: Mention that the pins should be at least 10 cm apart for a more accurate measurement.

Question 7: A ray of light falls normally on a plane mirror. What is the angle of incidence ?
Answer: Angle of incidence is 0°. Since angle of incidence is the angle between incident ray and normal. Direction of reflected ray is along BA opposite to the direction of incident ray.
If a ray falls 'normally,' it is traveling right along the Normal line itself. Because the ray and the normal are on top of each other, the gap (angle) between them is zero.
Teacher's Tip: "Normal" = 90° to the mirror, but 0° to the Normal line itself!
Exam Tip: Don't write 90°! Incidence is always measured from the Normal, so if it's on the Normal, it's 0°.

Question 8: Draw a diagram to show the reflection of a light ray incident normally on a plane mirror.
Answer: AO is the incident ray, OB is the reflected ray, ON is the normal, ∠ AON is the angle of incidence, ∠ NOB is the angle of reflection.
The diagram shows a straight vertical line hitting the mirror at a 90° angle. Arrows are drawn on the same line pointing both down (incident) and up (reflected).
Teacher's Tip: This is like a ball hitting a wall perfectly straight; it comes back exactly the way it went.
Exam Tip: Use two different colored arrows or offset them slightly so the teacher can see both rays.

Question 9: The diagram in Fig. shows an incident ray AO and the reflected ray OB from a plane mirror. The angle AOB is 30°. Draw normal on the plane mirror at the point O and find : (i) the angle of incidence (ii) the angle of reflection.
Answer: ON is normal on the plane mirror at point O. ON is perpendicular on a plane mirror. Angle of incidence
∠ i = ∠ AON and angle of reflection
∠ r = ∠ BON. Since, ∠ i = ∠ r and ∠ AOB = 30°
\implies ∠ AON + ∠ BON = 30°
\implies ∠ i + ∠ i = 30°
\implies 2 ∠ i = 30°
\implies ∠ i = 30 / 2 = 15°. ∴Angle of incidence = ∠ i = 15° and Angle of reflection = 15°.
Because the total opening between the two rays is 30°, the normal line divides it into two equal halves. Each half represents one of the angles required by the laws of reflection.
Teacher's Tip: The Normal is a 'bisector'—it cuts the big angle into two identical small ones.
Exam Tip: Show the calculation "30 / 2" to prove how you got your answer.

Question 10: In the following diagrams, measure and write the angle of incidence and draw the reflected ray in each case.
Answer: (a) ∠ i = 90° (Note: This is 0° from normal, 90° from surface), ∠ r = 90°. (b) ∠ i = 30°, ∠ r = 30°. (c) ∠ i = 50°, ∠ r = 50°.
In diagram (a), the ray hits the surface directly, bouncing back. In (b) and (c), the ray hits at an angle, so we draw a matching angle on the other side of the dashed normal line.
Teacher's Tip: Use a protractor to make sure your reflected ray is at the exact same angle as the incident one.
Exam Tip: Always draw the Normal line first if it's missing; it's your guide for 'r'.

Question 11: The diagram in fig. shows an incident ray AO and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray.
Answer: Law of reflection of light is used to draw the direction of the reflected ray. This law states that angle of incidence is equal to the angle of reflection. ∠ i = ∠ r.
If the incident ray is at 60° from the normal, we measure 60° on the opposite side of the normal and draw the reflected ray. This ensures the bounce is symmetrical.
Teacher's Tip: 'Symmetry' is the secret word for reflection!
Exam Tip: Label the angles as 60° on both sides of the Normal in your drawing.

Question 12: The following diagram shows an incident ray AO and the normal ON on a plane mirror. Find the angle of incidence and angle of reflection.
Answer: ON is perpendicular on a plane mirror. Angle of incidence ∠ i (∠ AON) i.e. Angle between incident ray and normal ray = 90° - 30° = 60°. Angle of Reflection = 60°.
∴ ∠ i = ∠ r.
∴ Angle between incident and reflected ray i.e. ∠ AOB = 60 + 60 = 120°.
The diagram shows the ray is 30° from the mirror surface. Since the Normal is at 90°, we subtract 30 from 90 to get the 60° incidence angle.
Teacher's Tip: The "Glancing Angle" (with the mirror) and "Incident Angle" (with the Normal) always add up to 90°.
Exam Tip: Never use the angle with the mirror as your angle of incidence; always subtract from 90° first.

Question 13: State in words, how do you find the location of image of an object formed by a plane mirror.
Answer: The location of image of a point object is as far behind the mirror as the object is in front of it.
To find it, you measure the perpendicular distance from the object to the mirror surface. Then, you mark a point at that same distance on the opposite side (behind the mirror).
Teacher's Tip: The mirror acts like a "midpoint" between you and your image.
Exam Tip: Use the phrase "perpendicular distance" to describe the measurement for extra marks.

Question 14: Draw a ray diagram showing the formation of image of a point object by a plane mirror.
Answer: The diagram shows a point 'object' in front of the mirror. Two rays are drawn hitting the mirror, then reflecting. When we extend those reflected rays backward with dashed lines, they meet at a point called 'Image.'
By showing two rays, we can pinpoint the exact spot where the image appears to be. The rays behind the mirror are dashed because they aren't real light rays.
Teacher's Tip: You need at least TWO rays to find an image point!
Exam Tip: Make sure the distance from object to mirror looks exactly the same as mirror to image.

Question 15: The following diagram shows a point object O placed in front of a plane mirror. Take two rays from the point O and show how the image of O is formed and seen by the eye.
Answer: From object O, draw two incident rays to the mirror. Reflect them following i=r toward the eye. Produce the reflected rays backward behind the mirror using dotted lines. The point where they meet is the virtual image of O.
This diagram explains why we see things "inside" the mirror. Our brain assumes light travels in a single straight line, so it follows the reflected rays back to the imaginary meeting point.
Teacher's Tip: The eye 'thinks' the light is coming from behind the mirror!
Exam Tip: Draw the eye at the end of the reflected rays to show someone is actually looking at the image.

Question 16: State four characteristics of the image formed by a plane mirror.
Answer: (i) The image formed is erect. (ii) The image is of the same size as that of the object. (iii) The image is laterally inverted. Right side appears to be left and left side appeared to be right. (iv) The image is virtual and is formed as far behind the mirror as the object is in front of it.
A plane mirror is perfectly faithful to size and distance but swaps your left and right. It doesn't create a real physical object, which is why we call the image virtual.
Teacher's Tip: Remember "S-V-E-L": Same size, Virtual, Erect, Laterally inverted.
Exam Tip: Listing these four characteristics is a common 4-mark question; memorize them as a set.

Question 17: How is the position of image formed by a plane mirror related to the position of the object ?
Answer: The image formed by a plane mirror is laterally inverted, upright, of the same size and is formed far behind the mirror as the object is in front of it.
The relationship is one of perfect distance matching. if the object is x distance away, the image is also x distance deep into the mirror world.
Teacher's Tip: If the object moves 1 step closer, the image also moves 1 step closer.
Exam Tip: Use the word "equidistant" to say both distances are equal.

Question 18: You are standing at a distance 2 metre from a plane mirror. (a) What is the distance of your image from the mirror ? (b) What is the distance between you and your image ?
Answer: (a) Distance of image from the mirror is also 2 metre. (b) Distance between me and my image is 4 metre.
Part (a) uses the rule that object distance equals image distance. Part (b) adds both distances together (2m + 2m = 4m) to find the total gap from the real person to their reflection.
Teacher's Tip: The image is 2m behind the glass, and you are 2m in front. That's a 4m total gap!
Exam Tip: Read carefully—if it asks for distance from "you," it means the total (Object + Image).

Question 19: What is meant by lateral inversion of an image in a plane mirror ? Explain it with the help of a diagram.
Answer: Lateral Inversion : Inter change of sides between the object and its image is called Lateral Inversion. Example : It means image formed behind the mirror is as far behind the mirror as object is in front of it. i.e. distance of M = dist of M in distance of O in front of mirror = distance of O image, and so on.
When you hold up a sign saying 'ATOM,' the mirror shows it as 'MOTA' (swapped). This happens because the light from the left side of the object stays on the left path, but because we are facing the mirror, it appears to be the 'right' of the image.
Teacher's Tip: This is why 'AMBULANCE' is written backward on the front of the vehicle!
Exam Tip: Draw the letter 'P' or 'B' to show lateral inversion clearly in your diagram.

Question 20: Write down the letter C and I as seen in a plane mirror.
Answer: (The letter C appears flipped backward, like a horseshoe opening to the left. The letter I stays exactly the same).
Symmetrical letters like 'I' look identical after lateral inversion. Asymmetrical letters like 'C' show the flip clearly because their left and right sides are different.
Teacher's Tip: Letters like A, H, I, M, O, T, U, V, W, X, Y are mirror-proof!
Exam Tip: Draw both letters carefully; 'I' is easy, but 'C' must be perfectly mirrored.

Question 21: What is irregular reflection ? Give an example.
Answer: Irregular reflection - When a beam of light falls on such a surface which is not perfectly smooth and polished such as wall, wood, paper etc. the different portions of the surface reflect light in different directions. Such a reflection of light from an uneven surface is called the irregular or diffused reflection.
In this type of reflection, parallel rays that hit the surface don't stay parallel when they bounce off. This scattering is what allows us to see non-shiny objects from many different positions.
Teacher's Tip: Irregular reflection is the reason a room is bright even if the sun isn't shining directly on every wall.
Exam Tip: Use the term "Diffused Reflection" as a synonym for irregular reflection.

Question 22: How do we see objects around us ?
Answer: Objects are seen when light after striking them, returns in the same medium and reach our eyes.
Most of the light we see is reflected light. For non-luminous objects, we rely on them bouncing light from a source (like a bulb or the sun) into our pupils.
Teacher's Tip: Eyes are 'receivers'; they only 'see' when light rays physically enter them.
Exam Tip: Mention that light must be reflected from the object and *reach our eyes* to be seen.

Question 23: State two uses of a plane mirror.
Answer: It is used as : (i) Looking glass (ii) In periscopes.
We use it every day to check our appearance (looking glass). In periscopes, mirrors are tilted at 45° to help people see around corners or above obstacles.
Teacher's Tip: A periscope uses TWO mirrors to bounce light in a 'Z' shape.
Exam Tip: You can also mention its use in solar cookers to reflect sunlight onto a pot.

Question 24: Can light travel in vacuum ?
Answer: Yes, light can travel in vacuum or air, a distance of nearly 299, 792, 458 metre (or nearly 3 x10⁸ metre) in one second. Thus, the speed of light in vacuum (or air) is 3 x 10⁸ m s^-1 nearly.
Unlike sound, light does not need a material (like air or water) to travel through. This is why we can see light from the sun and stars through the vast emptiness of space.
Teacher's Tip: If light couldn't travel in a vacuum, we would live in total darkness!
Exam Tip: Always provide the value 3 x10⁸ \text{ ms^-1 when asked about light in a vacuum.

Question 25: State the speed of light in (a) air, (b) glass.
Answer: 
(a) Air — 3 × 10⁸
(b) Glass — 2 × 10⁸
Light is slower in glass because the glass particles are more tightly packed together, creating more "resistance." Air is almost as empty as a vacuum, so light travels at full speed there.
Teacher's Tip: Air is fast, Water is medium, Glass is slow.
Exam Tip: Memorize these two values as they are frequently used in refraction problems.

Question 26: State whether light slows down or speeds up in the following cases : (a) Light going from air to glass. (b) Light going from glass to water. (c) Light going from water to air.
Answer: (a) Slows down (b) Speeds up (c) Speeds up
Light slows down when moving from a less dense material (like air) to a more dense one (like glass). It speeds up when moving into a less dense material because there is more 'room' to move.
Teacher's Tip: Less dense = Faster; More dense = Slower.
Exam Tip: 'Dense' in light science refers to "Optical Density," not how heavy the object is.

Question 27: What are the primary colours ? Name the three primary colours.
Answer: Primary colours are the colours of light by mixing which white light is obtained. They are : (i) red (ii) green and (iii) blue. Red + Green + Blue = White
These are the fundamental building blocks of light. Your eyes have special receptors for just these three colors, and your brain combines them to see millions of others.
Teacher's Tip: Just remember your computer screen pixels: R, G, and B.
Exam Tip: Make sure to mention that mixing all three primary light colors results in "White Light."

Question 28: What are the secondary colours ? Name the three secondary colours.
Answer: Secondary colours are the colours of light which are obtained by mixing the two primary colours. They are (i) yellow, (ii) cyan, and (iii) magenta.
For example, if you overlap red light and green light on a wall, the spot will appear yellow. This is known as the additive mixing of colors.
Teacher's Tip: Magenta looks like pinkish-purple, and Cyan looks like bright sky-blue.
Exam Tip: Be able to write the 'equations' (e.g., Red + Green = Yellow).

Question 29: Fill in the blanks with the appropriate colour
(a) Blue + ............ = Cyan
(b) Red + Blue + ............ = White
(c) Red + Blue = ............
(d) Green + Red = ............
Answer: (a) Blue + Green = Cyan (b) Red + Blue + Green = White (c) Red + Blue = Magenta (d) Green + Red = Yellow
This exercise tests your knowledge of color addition. Mixing these primary light beams creates new secondary colors or completes the full spectrum of white light.
Teacher's Tip: Use a "Color Triangle" diagram to help you memorize these combinations.
Exam Tip: If you forget the combinations, remember that primary + primary = secondary.

Question 30: The leaves appear green when seen in white light. Give a reason.
Answer: Leaves appear green in white light because they reflect only the green light and absorb the light of all the other colours.
White light contains every color (VIBGYOR). The chlorophyll in the leaf takes in all colors to make food but "rejects" the green part, which then enters our eyes.
Teacher's Tip: We see colors because objects 'spit out' that specific light toward us.
Exam Tip: Use both "reflect" and "absorb" to explain why we see a specific color.

Question 31: A rose appears red in white light. How will it appear in (i) green light, (ii) red light ? Give a reason for your answer for each.
Answer: (i) If a red rose is seen in green light, it appears black. The reason is that the rose absorbs the green light falling on it and reflect none. (ii) If a red rose is seen in red light, it appears bright red. This is because the rose reflects the red light falling on it and absorbs none of it.
This shows that an object's appearance depends on what light is hitting it. Without red light in the environment, a red object has nothing to reflect and essentially stays 'dark.'
Teacher's Tip: No reflection = No color = Blackness.
Exam Tip: State both the appearance (color) and the reason (reflection/absorption) for each part.

Question 32: Why does a piece of paper appear white in sunlight ? How would you expect it to appear when viewed in red light?
Answer: A piece of paper appears white in sunlight because it reflects light of all the colours. It would appear red when viewed in red light.
White surfaces are excellent reflectors that don't absorb any specific color. Therefore, they take on the color of whatever light shines on them.
Teacher's Tip: White paper is like a mirror that scatters light; it just copies the light color.
Exam Tip: Explain that a white object reflects "all components of white light."

Question 33: A piece of paper appears black in sunlight. What will be its colour when seen in red light ?
Answer: A piece of paper appear black in sunlight. It would appear black when seen in red light because it absorbs light of all the colours.
Since black objects are perfect absorbers, they won't reflect red light either. No matter what color you shine on black paper, it will stay black because it keeps all the light energy for itself.
Teacher's Tip: Black objects are 'light-greedy'—they never share light back with your eyes!
Exam Tip: Clarify that black paper absorbs "all colors," which includes red light.