Class 7 Science Light Exam Notes

Class 7 Science Light Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Following topics are covered:

These CBSE Class 7 Science exam notes on Light cover sources of light, the medium of light, rectilinear propagation, reflection and the laws of reflection, plane and spherical mirrors, refraction, spherical lenses, total internal reflection, dispersion by a glass prism, the human eye and its defects, and natural phenomena such as the rainbow, twinkling of stars, mirages and the blue colour of the sky. The chapter exercise with its answer key is included at the end.

Topics Covered

Light, sources of light, medium of light, linear propagation of light, behaviour of light, reflection of light, laws of reflection of light, regular and diffused reflection, reflection from plane mirror, reflection from spherical mirror, image formation by spherical mirror, refraction of light, spherical lens, image formation by spherical lens, total internal reflection, dispersion of white light by a glass prism, human eye, defects of vision, refraction in nature, scattering of light.

Light

Light is a form of energy (optical energy) which helps us see objects by its presence.
Light travels in a straight line with a speed of 3 × 10⁸ m/s in vacuum.
The velocity of light changes when it travels from one medium to another.
Light gets reflected back from polished surfaces, such as mirrors and polished metal surfaces.
Light undergoes refraction (bending) when it travels from one transparent medium to another.

Sources of Light

Luminous objects emit (give out) light. They may be natural or man-made. The Sun is a natural source of light, while an electric lamp and an oil lamp are man-made sources of light.

Non-luminous objects do not emit light. They become visible due to the reflection of the light falling on them. The Moon does not emit light; it becomes visible due to the reflection of sunlight falling on it.

Medium of Light

A substance through which light propagates, or tends to propagate, is called a medium of light. According to the medium of light, objects are divided into three types.

(i) Transparent objects: bodies that allow light to pass through them, so that light is transmitted through them. Examples: glass, water, air.
(ii) Translucent objects: bodies that transmit only a part of the light through them. Examples: frosted or ground glass, greased paper, paraffin wax.
(iii) Opaque objects: bodies that do not allow light to pass through them at all. Examples: chair, desk, wood.

Linear Propagation of Light

Light travels in a straight line. In vacuum or air, light travels with a speed of 3 × 10⁸ m/s.

Activity: If we look at a lighted candle through a straight pipe, we can see the candle. If we look at the candle through a bent pipe, we cannot see the candle flame. This activity shows that light travels along straight lines.

[Figure: (a) A straight pipe through which the lighted candle is visible to the eye. (b) A bent pipe through which the candle flame cannot be seen.]

Behaviour of Light

When light travelling in one medium falls on the surface of a second medium, the following three effects may occur:
(i) A part of the incident light is turned back into the first medium. This is called reflection of light.
(ii) A part of the incident light is transmitted into the second medium along a changed direction. This is called refraction of light.
(iii) The remaining part of the light energy is absorbed by the second medium. This is called absorption of light.

[Figure: Incident light falling on the boundary between Medium 1 and Medium 2, with the reflected light going back into Medium 1 and the refracted light going into Medium 2.]

Reflection of Light

When light rays are incident on an opaque polished surface, they are returned back into the same medium. This phenomenon is called reflection of light.

Types of mirror: plane mirror and spherical mirror. Spherical mirrors are of two types: convex and concave.

Some Associated Terms

Reflecting surface: the surface from which light is reflected. In the diagram, XY is the reflecting surface. (XY is the section of the reflecting surface made by the plane of the page, which is perpendicular to it.)
Point of incidence: the point on the reflecting surface at which a ray of light strikes. In the diagram, O is the point of incidence.
Normal: a perpendicular drawn on the reflecting surface at the point of incidence. In the diagram, NO is the normal.
Incident ray: the ray of light which strikes the reflecting surface at the point of incidence. In the diagram, PO is the incident ray.
Reflected ray: the ray of light reflected from the point of incidence. In the diagram, OQ is the reflected ray.
Angle of incidence (i): the angle that the incident ray makes with the normal. In the diagram, angle PON is the angle of incidence.
Angle of reflection (r): the angle that the reflected ray makes with the normal. In the diagram, angle QON is the angle of reflection.

[Figure: Reflecting surface XY, normal NO at point of incidence O, incident ray PO, reflected ray OQ, angle of incidence i and angle of reflection r.]

Laws of Reflection of Light

First law: the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Second law: the angle of reflection (r) is always equal to the angle of incidence (i), that is, r = i.
For normal incidence, i = 0 and r = 0, so the ray is reflected back along the normal.

Regular and Diffused Reflection

Regular reflection: a parallel beam of light remains parallel after reflection from a plane smooth surface. This reflection follows the laws of reflection.

[Figure: Regular reflection of a parallel beam from a plane smooth surface.]

Irregular or diffused reflection: a parallel beam of light is reflected in random directions from a rough surface. This reflection also follows the laws of reflection, because each ray obeys the law at its own point of incidence.

[Figure: Irregular or diffused reflection of a parallel beam from a rough surface.]

Reflection from Plane Mirror

Size: the size of the image is the same as that of the object.
Upright or erect: the image is erect with respect to the object.
Image distance: the image distance is the same as the object distance.
Lateral inversion: if you move your right hand, the image appears to move its left hand. If you hold a printed page in front of a plane mirror, the letters of the image appear erect but inverted sideways. This sideways reversal is called lateral inversion.

[Figure: Object showing L and R, and its image in a plane mirror showing R and L, illustrating lateral inversion.]

Reflection from Spherical Mirror

There are two types of spherical mirrors:
(i) Concave mirror: the reflection takes place from the inner (curved) surface.
(ii) Convex mirror: the reflection takes place from the outer (bulging) surface.

[Figure: (i) Concave mirror with centre of curvature C, focus F, pole P, principal axis and points A and B on the mirror. (ii) Convex mirror with the same labels.]

Image Formation by Spherical Mirror

Ray diagram for image formation from a concave mirror:
(a) A ray of light incident parallel to the principal axis is reflected through the focus F.
(b) A ray of light incident towards the focus F is reflected parallel to the principal axis.
(c) A ray of light incident towards the centre of curvature C is reflected back along the same path.
(d) A ray of light incident on the pole P is reflected symmetrically, making the same angle with the principal axis.

[Figure: Four ray diagrams for a concave mirror with centre of curvature C, focus F and pole P, showing the rays described in (a) to (d).]

Rules for image formation from a convex mirror:
(a) A ray of light incident parallel to the principal axis is reflected as if it came from the focus F behind the mirror.
(b) A ray of light incident on the mirror directed towards the centre of curvature C (behind the mirror) is reflected back along the same path.

OR (convex mirror, alternative rays): a ray of light incident towards the focus F behind the mirror is reflected parallel to the principal axis. Similarly, a ray incident parallel to the principal axis is reflected as if it came from the focus F behind the mirror. In the figure the rays meeting the mirror at 90° are directed towards C behind the mirror.

[Figure: Convex mirror with pole P, focus F and centre of curvature C behind the mirror, showing the reflected rays for (a) and (b).]

Refraction of Light

When light rays travelling in a medium are incident on a transparent surface of another medium, they are bent as they travel in the second medium. When light passes from a rarer medium to a denser medium, it bends towards the normal.

[Figure: Refraction of a ray at a plane transparent surface, showing the incident ray in the rarer medium, the normal N1 N2, the angle of incidence i, the angle of refraction r in the denser medium, and the refracted ray.]

Spherical Lens

A piece of a transparent medium bounded by at least one spherical surface is called a spherical lens.

There are two types of spherical lenses:
(i) Convex or converging lens: thick in the middle and thin at the edges.
(ii) Concave or diverging lens: thin in the middle and thick at the edges.

Image Formation by Spherical Lens

Ray diagram for image formation by a convex lens:
(a) A ray of light incident parallel to the principal axis passes through the focus F2 after refraction.
(b) A ray of light incident through the focus F1 emerges parallel to the principal axis.
(c) A ray of light incident on the optical centre passes straight through without deviation.

[Figure: Convex lens with optical centre, principal axis, foci F1 and F2 on either side, and the three rays (a), (b) and (c).]

Ray diagram for image formation by a concave lens:
A ray of light incident parallel to the principal axis is refracted so that it appears to diverge from the focus F1 on the same side as the incident ray.

A ray of light directed towards the focus on the other side of the lens (F1) is refracted by the concave lens so that it emerges parallel to the principal axis.

A ray of light incident on the optical centre C passes straight through the lens without deviation.

[Figure: Concave lens with foci F1 and F2 and rays incident parallel to the principal axis, diverging after refraction.]

Total Internal Reflection

When light travels from a denser medium to a rarer medium and is incident at an angle greater than the critical angle for that medium, it is completely reflected back into the denser medium. This complete inward reflection of light is called total internal reflection.

[Figure: Total internal reflection of a ray from water into air, with the incident ray at angles i1 to i4 and the totally reflected ray inside the water.]

Dispersion of White Light by a Glass Prism

Definition: when a ray of white light (sunlight) enters a glass prism (a denser medium), it emerges broken into seven colours. The separation of the different components of white light by a denser medium is called dispersion.

Explanation: dispersion happens because light of different colours travels with different speeds in a denser medium. White light has seven colours: violet, indigo, blue, green, yellow, orange and red, which can be remembered by the word VIBGYOR. In air (strictly in vacuum), all colours travel with the same speed, 3 × 10⁸ m/s. In a denser medium their speeds become smaller and different. Red light travels fastest and has the maximum speed, while violet light travels slowest and has the minimum speed. Because of different deviations, the colours emerge in different directions.

When the dispersed white light falls on a white screen, a seven-coloured band is seen. This coloured band is called a spectrum.

[Figure: A ray of white light entering a glass prism at A and emerging as a spectrum of seven colours, from red at the top to violet at the bottom, falling on a screen.]

Human Eye

The human eye is the most delicate and complicated natural optical instrument. It is a spherical ball of diameter about 2.5 cm. Its main parts are:
1. Cornea: the transparent front part of the eye.
2. Aqueous humour: a watery liquid behind the cornea.
3. Iris: the coloured part of the eye, which controls the size of the pupil.
4. Pupil: the opening in the iris through which light enters.
5. Eye lens: a convex lens that focuses light on the retina.
6. Ciliary muscle: muscles that change the focal length of the eye lens.
7. Vitreous humour: a jelly-like liquid that fills the eyeball.
8. Retina: the screen on which the image is formed. It contains the light-sensitive cells.
9. Yellow spot (fovea): the part of the retina where the image is most sharp.
10. Optic nerve: carries signals from the retina to the brain.
11. Blind spot: the part of the retina where the optic nerve leaves the eye, which has no light-sensitive cells.
12. Sclerotic: the white outer covering of the eye.
13. Choroid: the dark layer below the sclerotic.

[Figure: Horizontal section of the human eye showing sclerotic (white of eye), choroid (black), cornea, iris, pupil, aqueous humour, ciliary muscle, eye lens, vitreous humour, retina, yellow spot (fovea), optic nerve and blind spot.]

Defects of Vision

The major defects of vision are:
1. Short-sightedness or myopia
2. Long-sightedness or hypermetropia

1. Short-sightedness or myopia
Symptoms: the eye cannot see objects clearly beyond a certain distance. The far point of the defective eye has shifted from infinity to a finite distance in front of the eye.
Correction: the extra converging power of the eye lens is compensated by using a concave (diverging) lens of proper focal length.

[Figure: Myopia corrected by a concave lens placed in front of the eye, with foci F and F' marked.]

2. Long-sightedness or hypermetropia
Symptoms: the eye cannot see objects clearly within a certain distance. The near point of the defective eye has shifted from 25 cm to a greater distance.
Correction: the deficiency in the converging power of the eye lens is compensated by using a convex (converging) lens of proper focal length.

[Figure: Hypermetropia corrected by a convex lens placed in front of the eye, with the near point N and the distance D of 25 cm marked.]

Refraction in Nature

(A) Formation of rainbow: a rainbow is a natural spectrum that appears in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets in the atmosphere. A rainbow is always formed in the direction opposite to the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, reflect it internally, and then refract it again as it comes out of the raindrop. Because of dispersion and internal reflection, the different colours reach the observer's eye.

[Figure: Sunlight entering a raindrop, dispersing into red and violet, reflecting internally, and emerging towards the observer, with the horizontal line of sight shown.]

(B) Atmospheric refraction: the apparent random wavering or flickering of objects seen through a turbulent stream of hot air, such as the air rising above a fire or a radiator, is an effect of atmospheric refraction. The air just above the fire is hotter, lighter and less dense than the air above it, so it has a slightly smaller refractive index. Since the air is not stationary, the apparent position of the object fluctuates. The twinkling of stars is a similar phenomenon on a much larger scale.

(a) Twinkling of stars: starlight undergoes refraction continuously as it passes through the earth's atmosphere, which has a gradually changing refractive index. Stars are very distant, so they are almost point-sized sources of light. As the path of the light changes slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers. So the star sometimes appears brighter and sometimes fainter. This is the twinkling effect.

(b) Why do planets not twinkle? Planets are much closer to the earth and are seen as extended sources, which is a collection of many point-sized sources of light. The variations in the light from all these point sources average out to zero, so the twinkling effect is nullified.

(C) Advance sunrise and delayed sunset: the Sun is visible about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset, because of atmospheric refraction. Actual sunrise means the actual crossing of the horizon by the Sun. The apparent flattening of the Sun's disc at sunrise and sunset is also due to atmospheric refraction.

[Figure: Earth with its atmosphere, an observer and the horizon, showing the actual position of the Sun and its apparent position due to atmospheric refraction at sunrise and sunset.]

Scattering of Light

The interaction of light with objects around us gives rise to many spectacular phenomena in nature, such as the blue colour of the sky, the colour of water in deep sea, and the reddening of the Sun at sunrise and sunset. The path of a beam of light passing through a true solution is not visible. However, its path becomes visible through a colloidal solution, where the size of the particles is relatively larger.

(a) Tyndall effect: the earth's atmosphere is a heterogeneous mixture of minute particles, including smoke, tiny water droplets, dust particles and air molecules. When a beam of light strikes such fine particles, its path becomes visible, because the light is scattered (reflected diffusely) by the particles. This scattering of light by colloidal particles is called the Tyndall effect. It is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. It can also be seen when sunlight passes through the canopy of a dense forest, where tiny water droplets in the mist scatter light. The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light, while larger particles scatter light of longer wavelengths. If the particles are large enough, the scattered light may appear white.

(b) Why is the colour of the clear sky blue? The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. They scatter light of shorter wavelengths (blue end) much more strongly than light of longer wavelengths (red end). Red light has a wavelength about 1.8 times greater than blue light. So when sunlight passes through the atmosphere, the blue colour is scattered more strongly, and this scattered blue light enters our eyes. If the earth had no atmosphere, there would be no scattering and the sky would look dark. The sky appears dark to passengers flying at very high altitudes, because scattering is not prominent at such heights. Danger signal lights are red because red light is least scattered by fog or smoke, so it can be seen in the same colour from a distance.

(c) Colour of the Sun at sunrise and sunset: at sunrise and sunset the Sun appears red. Near the horizon, most of the blue light and other shorter wavelengths are scattered away by the particles in the air. So the light that reaches our eyes has longer wavelengths, which gives the Sun its reddish appearance.

[Figure: Sun near the horizon, with blue light scattered away and less blue light reaching the observer, so that the Sun appears reddish. Sun nearly overhead is shown for comparison.]

Mirage or Inferior Mirage

A mirage is an optical illusion seen in deserts at summer noon. An inverted image of a distant tree appears to be formed in the hot sand below it, as if it were formed in water. In fact, there is no water anywhere.

Explanation: the phenomenon is due to total internal reflection. At summer noon the sand becomes very hot, and the air in contact with it becomes very hot and rarer. As we move up, the air becomes less and less hot, hence less and less rarer. So the air forms layers of different optical density. A ray from the top of the tree that makes a small angle with the vertical reaches the lowest layer at an angle that may not be more than the critical angle, so it is absorbed by the sand. A ray that makes a bigger angle reaches the lowest layer at an angle just more than the critical angle. It is totally reflected upward and outward towards the observer. All rays to the right of this ray also make larger angles and are totally reflected in the same way.

[Figure: Hot desert sand under layers of air of decreasing temperature, a tree above, and the observer far away. Ray 1 is absorbed by the sand, ray 2 is totally reflected upward, and the inverted image of the tree appears in the hot sand.]

Looming or Superior Mirage

Looming is an optical illusion seen at the seashore on a winter evening, in which an image of a ship appears to be formed in the air between the sea and the sky. The actual ship is not visible.

Explanation: the phenomenon is due to total internal reflection. On a cold evening, the sea water becomes very cold, and the air in contact with it becomes cold and denser. Higher up, the air layers become less cold and rarer. Rays from the invisible ship travel upward from denser to rarer air layers. They are totally reflected downwards and received by an observer at the seashore. The observer sees a virtual image of the ship hanging in the sky.

[Figure: Observer at the sea shore, cold sea water below layers of air becoming less cold and rarer upward, a ship below the horizon whose virtual image appears in the sky.]

Exercise

Single Correct Answer Type Questions

1. Which of these form virtual images only?
(A) Concave mirror
(B) Convex mirror
(C) Convex lens
(D) None of these

Answer: (B) Convex mirror

1. A convex mirror always forms a virtual, erect and diminished image.
2. A concave mirror and a convex lens can form real images as well as virtual images.
3. Hence, the correct option is (B).

Teacher's Note:
a) A virtual image cannot be caught on a screen.
b) Check whether the mirror is convex or concave before deciding the image type.

2. A drop of water on a leaf forms a magnified image of the veins because of-
(A) refraction
(B) reflection
(C) radiation
(D) rectilinear propagation

Answer: (A) refraction

1. A drop of water acts like a small convex lens.
2. A convex lens magnifies objects by refraction of light.
3. Hence, the correct option is (A).

Teacher's Note:
a) Magnification by a convex lens is due to refraction, not reflection.
b) The drop bends the light rays that pass through it.

3. If we mix lights of the colours of the rainbow, we will get
(A) pink light
(B) brown light
(C) colourless light
(D) black light

Answer: (C) colourless light

1. The colours of the rainbow are the seven colours of white light.
2. When all the seven colours are mixed again, white (colourless) light is formed.
3. Hence, the correct option is (C).

Teacher's Note:
a) This is the reverse of dispersion by a prism.
b) Mixing coloured paints does not give the same result as mixing coloured lights.

4. If you bring a faraway object towards the focus of convex lens, the size of the image will-
(A) increase
(B) decrease
(C) double
(D) remain the same

Answer: (A) increase

1. A faraway object forms a very small image at the focus of the convex lens.
2. As the object moves towards the focus, the image moves away from the lens and becomes larger.
3. Hence, the correct option is (A).

Teacher's Note:
a) The image size grows as the object comes closer to the focus.
b) The image is real and formed on the other side of the lens.

5. Which of these are due to the rectilinear propagation of light?
(A) rainbow
(B) inverted image in a pinhole camera
(C) shadow
(D) reflection

Answer: (C) shadow

1. A shadow is formed because light travels in a straight line and is blocked by an object.
2. The inverted image in a pinhole camera is also formed by the straight-line travel of light.
3. Hence, the key option is (C).

Teacher's Note:
a) Both the shadow and the pinhole image are due to rectilinear propagation; the printed options allow only one choice.
b) A rainbow is due to dispersion, and reflection is a separate phenomenon.

6. Light causes the sensation of-
(A) Vision
(B) Light
(C) Both (A) and (B)
(D) None

Answer: (A) Vision

1. Light falling on the retina produces the sensation of vision.
2. Hence, the correct option is (A).

Teacher's Note:
a) Light itself is the stimulus, and vision is the sensation it produces.
b) Without light the eye cannot see objects.

7. Light is-
(A) an electromagnetic radiation
(B) a longitudinal wave
(C) massless
(D) all of the above

Answer: (A) an electromagnetic radiation

1. Light is an electromagnetic wave that does not need a medium to travel.
2. Light is a transverse wave, not a longitudinal wave, and it has no mass.
3. Hence, the correct option is (A).

Teacher's Note:
a) Electromagnetic waves are transverse.
b) Light is one of many electromagnetic radiations.

8. Which of the following is a natural luminous source of light?
(A) sun
(B) wood
(C) electric lamp
(D) torch

Answer: (A) sun

1. The Sun produces its own light naturally.
2. Wood, electric lamps and torches are not natural luminous sources.
3. Hence, the correct option is (A).

Teacher's Note:
a) Wood is non-luminous and becomes visible by reflecting light.
b) Electric lamps and torches are man-made sources of light.

9. Light shows-
(A) random propagation
(B) curvilinear propagation
(C) rectilinear propagation
(D) None of these

Answer: (C) rectilinear propagation

1. Light travels in straight lines, which is called rectilinear propagation.
2. Hence, the correct option is (C).

Teacher's Note:
a) The bent pipe activity shows that light does not travel along curved paths.
b) Light is slightly bent when it passes through a medium of changing density, but it travels in straight lines in a uniform medium.

10. Which of the following is a reflector of light?
(A) Sun
(B) Star
(C) Filament
(D) Moon

Answer: (D) Moon

1. The Moon does not emit light.
2. It reflects sunlight falling on its surface, so it is a reflector of light.
3. Hence, the correct option is (D).

Teacher's Note:
a) The Sun, stars and the filament of a bulb are luminous sources.
b) A reflector sends back light that falls on it.

11. Wood is an example of-
(A) translucent
(B) Transparent
(C) Polymer
(D) Opaque

Answer: (D) Opaque

1. Ordinary wood does not allow light to pass through it.
2. Hence, wood is opaque, and the correct option is (D).

Teacher's Note:
a) Transparent objects such as glass allow light to pass through.
b) Translucent objects such as frosted glass allow only part of the light through.

12. If the angle of incidence is 50°, then calculate the angle between the incident ray and the reflected ray-
(A) 50°
(B) 80°
(C) 130°
(D) 100°

Answer: (D) 100°

1. Angle of reflection = angle of incidence = 50°.
2. The incident ray and the reflected ray are each 50° from the normal, on opposite sides of it.
3. Angle between the incident ray and the reflected ray = 50° + 50° = 100°.
4. Hence, the correct option is (D).

Teacher's Note:
a) The angle between the two rays is twice the angle of incidence.
b) Do not confuse this angle with the angle between the ray and the mirror surface, which is 40°.

13. Which of the following statement is true?
(A) The angle of incidence is twice the angle of reflection
(B) The incident ray, the reflected ray and the normal drawn at the same point of incidence lie in the same plane
(C) Some types of virtual images can be caught on the screen.
(D) A plane mirror forms a real image

Answer: (B) The incident ray, the reflected ray and the normal drawn at the same point of incidence lie in the same plane

1. This statement is the first law of reflection.
2. The angle of incidence equals the angle of reflection, so option (A) is false.
3. Virtual images cannot be caught on a screen, and a plane mirror forms a virtual image, so options (C) and (D) are false.
4. Hence, the correct option is (B).

Teacher's Note:
a) The first law of reflection concerns the plane of the rays and the normal.
b) Plane mirrors always form virtual images.

14. Two plane mirrors are inclined at a angle 60°, the number of images of an object which is placed between mirror will be-
(A) 4
(B) 3
(C) 5
(D) 6

Answer: (C) 5

1. Number of images n = \( \frac{360^\circ}{\theta} - 1 \).
2. n = \( \frac{360^\circ}{60^\circ} - 1 \) = 6 - 1 = 5.
3. Hence, the correct option is (C).

Teacher's Note:
a) The formula works when 360° divided by the angle is a whole number.
b) Each mirror forms its own images, and the images also reflect in the other mirror.

15. Plane mirror are arranged parallel to each other to get-
(A) A single image
(B) Two images
(C) A large number of reflected images
(D) No image

Answer: (C) A large number of reflected images

1. When two plane mirrors face each other parallel, each image is reflected again and again.
2. This gives a large number of reflected images, so the correct option is (C).

Teacher's Note:
a) The images are formed in a long series on both sides.
b) Parallel mirrors are used in lifts and barber shops for this reason.

16. When an object is moved towards the plane mirror-
(A) Image moves away from the object
(B) Size of the image increases
(C) Image moves closer to the object
(D) Size of the image decrease

Answer: (C) Image moves closer to the object

1. In a plane mirror the image is as far behind the mirror as the object is in front of it.
2. When the object moves towards the mirror, the image also moves towards the mirror.
3. So the distance between the object and its image becomes smaller, and the size of the image does not change. Hence, the correct option is (C).

Teacher's Note:
a) The size of the image in a plane mirror always equals the size of the object.
b) Both the object and the image move closer to the mirror at the same time.

17. David is observing his image in a plane mirror. The distance between the mirror and his image is 5m. If he moves 1m towards the mirror, then the distance between David and his image will be-
(A) 3 m
(B) 5 m
(C) 6 m
(D) 8 m

Answer: (D) 8 m

1. David is 5 m from the mirror, because the image is as far behind the mirror as David is in front of it.
2. After moving 1 m towards the mirror, David is 4 m from the mirror.
3. His image is then 4 m behind the mirror, so the distance between David and his image = 4 m + 4 m = 8 m.
4. Hence, the correct option is (D).

Teacher's Note:
a) The question's "distance between the mirror and his image" is taken as the distance of the image behind the mirror.
b) The distance between an object and its image is twice the object's distance from the mirror.

18. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be-
(A) 1 m/s
(B) 2 m/s
(C) 4 m/s
(D) 8 m/s

Answer: (B) 2 m/s

1. The truck is stationary, and the car moves backwards towards it at 2 m/s.
2. The mirror moves with the car, so the distance between the mirror and the truck decreases at 2 m/s.
3. The image is always as far behind the mirror as the truck is, so the image approaches the driver at 2 m/s.
4. Hence, the correct option is (B).

Teacher's Note:
a) The image moves at the same speed as the mirror, but it moves in the opposite direction relative to the mirror.
b) Add the speeds only when both the object and the mirror move towards each other.

19. For a concave mirror when the object is placed between the pole and the focus then the image formed will be-
(A) Virtual
(B) Real
(C) Inverted
(D) Diminished

Answer: (A) Virtual

1. When the object lies between the pole and the focus of a concave mirror, the reflected rays diverge.
2. The rays appear to come from behind the mirror, so the image is virtual, erect and magnified.
3. Hence, the correct option is (A).

Teacher's Note:
a) A virtual image cannot be formed on a screen.
b) The image is also erect and magnified in this position.

20. Mirror used to form magnified image is-
(A) Concave mirror
(B) Convex mirror
(C) Plane mirror
(D) None of these

Answer: (A) Concave mirror

1. A concave mirror forms a magnified image when the object is placed between the pole and the focus.
2. A convex mirror always gives a diminished image, and a plane mirror gives an image of the same size.
3. Hence, the correct option is (A).

Teacher's Note:
a) Dentists and ENT surgeons use concave mirrors for magnified views.
b) A plane mirror forms an image of the same size as the object.

21. A convex mirror always produces-
(A) an erect, real image of diminished size
(B) an erect, real image of enlarged size
(C) a virtual, erect image of enlarged size
(D) an erect, virtual image of diminished size

Answer: (D) an erect, virtual image of diminished size

1. A convex mirror diverges the reflected rays.
2. The image is always virtual, erect and smaller than the object.
3. Hence, the correct option is (D).

Teacher's Note:
a) A convex mirror gives a wider field of view, which is why it is used as a rear view mirror.
b) Option (C) describes an image that is not produced by a convex mirror.

22. A reflecting surface is curved inwards. Now the mirror formed is-
(A) concave
(B) plane
(C) convex
(D) none of these

Answer: (A) concave

1. A mirror whose reflecting surface curves inwards is a concave mirror.
2. Hence, the correct option is (A).

Teacher's Note:
a) Reflection takes place from the inner surface of a concave mirror.
b) A convex mirror is curved outwards.

23. The phenomenon of the change in the path of the light as it passes from one optical medium to another is called-
(A) Reflection of the light
(B) Refraction of light
(C) Dispersion of light
(D) Both (A) and (B)

Answer: (B) Refraction of light

1. Refraction is the bending of light when it passes from one optical medium into another.
2. Reflection sends light back into the same medium, and dispersion is the splitting of white light, so option (D) is not correct.
3. Hence, the correct option is (B).

Teacher's Note:
a) Refraction is caused by the change in the speed of light in the new medium.
b) Dispersion is a special case of refraction in which the colours separate.

24. Arrange the optical mediums in ascending order according to optically denser.
(A) Air, water, glass
(B) Water, glass, air
(C) Glass, water, air
(D) Glass, air, water

Answer: (A) Air, water, glass

1. Air is the least optically dense medium of the three.
2. Water is optically denser than air, and glass is optically denser than water.
3. So the ascending order of optical density is air, water, glass. Hence, the correct option is (A).

Teacher's Note:
a) Optical density is about how much the light slows down in a medium.
b) Optical density is not the same as mass density.

25. We put glass piece on a printed page image of prints on the page has same size. The piece is-
(A) Convex lens
(B) Glass slab
(C) Concave lens
(D) Prism

Answer: (B) Glass slab

1. A glass slab has parallel faces and does not magnify the image.
2. The print seen through it has the same size, so the piece is a glass slab. Hence, the correct option is (B).

Teacher's Note:
a) A convex or concave lens would change the size of the print.
b) A glass slab only shifts the image slightly.

26. What happens, when a ray incident at the optical centre?
(A) It passes with deviation of 30° angle through the lens
(B) It passes undeviated through the lens
(C) It passes with deviation of 45° angle through the lens
(D) None of these

Answer: (B) It passes undeviated through the lens

1. The optical centre of a lens is the point at which the two faces are effectively parallel.
2. A ray passing through the optical centre goes straight through without any deviation. Hence, the correct option is (B).

Teacher's Note:
a) The optical centre lies on the principal axis of the lens.
b) This rule is one of the three standard rays used in ray diagrams for lenses.

27. If the lower part of a convex lens is blackened then the image formed will be-
(A) incomplete
(B) complete
(C) of lower intensity
(D) both (B) and (C)

Answer: (D) both (B) and (C)

1. Every part of a lens forms a complete image of the object.
2. Blackening the lower part blocks some of the light, so the image is complete but of lower intensity.
3. Hence, the correct option is (D).

Teacher's Note:
a) The same principle applies to a mirror, where a covered half still forms a full image.
b) Brightness of the image falls because less light reaches the screen.

28. Which of the following diagrams correctly represent the passage of a ray of light through a concave lens ?

[Figure: Four diagrams of a concave lens with its principal axis and the focus F marked. (A) A ray parallel to the axis passes straight through the lens without bending. (B) A ray parallel to the axis bends downwards across the axis towards the point marked F on the right of the lens. (C) A ray bends downwards across the axis towards the point marked 2F on the right of the lens. (D) A ray parallel to the axis bends away from the axis, as if it came from the focus F.]

Answer: (D) Ray parallel to the principal axis bent away from the axis, as if coming from the focus F

1. A concave lens is a diverging lens.
2. A ray parallel to the principal axis is refracted so that it appears to come from the focus on the same side.
3. Hence, the correct option is (D).

Teacher's Note:
a) A concave lens always gives a diminished, virtual and erect image.
b) A ray passing through the optical centre is not deviated by any lens.

29. White light spectrum contains-
(A) 5 colours
(B) 7 colours
(C) 6 colours
(D) No colour

Answer: (B) 7 colours

1. White light is made up of seven colours: violet, indigo, blue, green, yellow, orange and red.
2. Hence, the correct option is (B).

Teacher's Note:
a) The word VIBGYOR helps to remember the seven colours.
b) Indigo is often hard to tell apart from blue and violet.

30. The diagram below shows two incident rays P and Q which emerge as parallel rays R and S. The appropriate device used in the box A is-
(A) convex lens
(B) concave lens
(C) prism
(D) concave mirror

[Figure: Two incident rays P and Q, drawn converging towards a box labelled A, emerge from the box as two parallel rays R and S.]

Answer: (B) concave lens

1. The incident rays P and Q are converging towards a point behind the box.
2. A concave lens bends converging rays so that they emerge parallel to each other.
3. Hence, the correct option is (B).

Teacher's Note:
a) A prism would change the directions of P and Q but would not make both emerge parallel.
b) A convex lens would bring the rays to a point, not make them parallel.

Answer Key

Exercise answers: 1 (B), 2 (A), 3 (C), 4 (A), 5 (C), 6 (A), 7 (A), 8 (A), 9 (C), 10 (D), 11 (D), 12 (D), 13 (B), 14 (C), 15 (C), 16 (C), 17 (D), 18 (B), 19 (A), 20 (A), 21 (D), 22 (A), 23 (B), 24 (A), 25 (B), 26 (B), 27 (D), 28 (D), 29 (B), 30 (B).

In the following videos we have explained you about the concept of Light and its properties. Watch this video to understand the concepts and also post your questions and comments in the comments section

(i) Transparent object :
Bodies that allow light to pass through then i.e. transmit light through them, are called transparent bodies. Ex. Glass, water, air etc

Revise these notes regularly and practise the exercise above to prepare well for your exams.