Light Energy

Objective Questions

Question 1

Write true or false for each statement:

(a) Water is optically denser than glass.

(b) A ray of light when passes from glass to air, bends towards the normal.

(c) The speed of light is more in glass than in water.

(d) The depth of a pond when seen from above appears to be less.

(e) Light travels at a lower speed in water than in air.

(f) Light travels in the same straight line path while passing through different media.

(g) The angle formed between the normal and the refracted ray is known as the angle of incidence.

(h) At the point of incidence, a line drawn at right angles to the surface, separating the two media, is called the normal.

(i) Image is formed by a mirror due to refraction of light.

(j) Rays of light incident parallel to the principal axis pass through the focus after reflection from a concave mirror.

(k) A convex mirror is used as a shaving mirror.

(l) The focal length of a convex mirror is equal to its radius of curvature.

(m) A concave mirror converges the light rays, but a convex mirror diverges them.

(n) A virtual image formed by a spherical mirror is always erect and situated behind the mirror.

Answer

(a) False
Correct Statement — Water is optically rarer than glass.

(b) False
Correct Statement — A ray of light when passes from glass to air, bends away from the normal.

(c) False
Correct Statement — The speed of light is less in glass than in water.

(d) True

(e) True

(f) False
Correct Statement — The direction of light changes from its initial path when passing through different media.

(g) False
Correct Statement — The angle formed between the normal and the refracted ray is known as the angle of refraction.

(h) True

(i) False
Correct Statement — Image is formed by a mirror due to reflection of light.

(j) True

(k) False
Correct Statement — A concave mirror is used as a shaving mirror.

(l) False
Correct Statement — The focal length of a convex mirror is half of its radius of curvature.

(m) True

(n) True

Question 2

Fill in the blanks:

(a) Water is optically ............... than air.

(b) Air is optically ............... than glass.

(c) When a ray of light travels from water to air, it bends ............... the normal.

(d) When a ray of light travels from air to glass, it bends ............... the normal.

(e) When white light passes through a prism, it ............... .

(f) The splitting of white light into its constituent colours is called ............... .

(g) A ............... mirror is obtained on silvering the outer surface of a part of a hollow glass sphere.

(h) Radius of curvature of a spherical mirror is ............... its focal length.

(i) The angle of incidence for a ray of light passing through the centre of curvature of a spherical mirror is ............... .

(j) A ............... mirror always forms a virtual image.

(k) A concave mirror forms a virtual image for an object placed ............... .

Answer

(a) denser

(b) rarer

(c) away from

(d) towards

(e) disperses

(f) dispersion

(g) concave

(h) two times

(i) 0°

(j) convex

(k) between pole and focus

Question 3

Match the following:

Answer

Question 4a

The speed of light in air or vacuum is:

1. 3 x 10⁸ m s^-1
2. 2.25 x 10 ⁸ m s^-1
3. 332 m s^-1
4. 2.0 x 10⁸ m s^-1

Answer

3 x 10⁸ m s^-1

Reason — Speed of light in air or vacuum is fastest i.e. 3 x 10⁸ m s^-1

Question 4b

A ray of light moving from an optically rarer to a denser medium:

1. bends away from the normal
2. bends towards the normal
3. remains undeviated
4. None of the above

Answer

bends towards the normal

Reason — When light ray passes from rarer to denser medium speed of light decreases so it bends towards the normal.

Question 4c

The angle between the normal and refracted ray is called:

1. angle of deviation
2. angle of incidence
3. angle of refraction
4. angle of emergence

Answer

angle of refraction

Reason — Angle of refraction is the angle formed between the normal and refracted ray.

Question 4d

The property of splitting of white light into its seven constituent colours is known as:

1. rectilinear propagation
2. refraction
3. reflection
4. dispersion

Answer

dispersion

Reason — Dispersion is the property of splitting of white light into its seven constituent colours.

Question 4e

The seven colours in the spectrum of sunlight in order are represented as:

1. VIBGYOR
2. VIGYBOR
3. BIVGYOR
4. RYOBIVG

Answer

VIBGYOR

Reason — The order of seven colours in the spectrum of sunlight is VIBGYOR i.e. violet, indigo, blue, green, yellow, orange and red, respectively.

Question 4f

A ray of light passing through the centre of curvature of a spherical mirror, after reflection:

1. passes through the focus
2. passes through the pole
3. becomes parallel to the principal axis
4. retraces its own path

Answer

retraces its own path

Reason — A ray of light passing through the centre of curvature is incident normally on the spherical mirror and its angle of incidence and angle of reflection is zero so the light ray retraces its own path.

Question 4g

If the radius of curvature of a concave mirror is 20 cm, its focal length is:

1. 10 cm
2. 20 cm
3. 40 cm
4. 80 cm

Answer

10 cm

Reason — Focal length is half of radius of curvature, so it will be 10 cm.

Question 4h

The image formed by a convex mirror is:

1. erect and diminished
2. erect and enlarged
3. inverted and diminished
4. inverted and enlarged

Answer

erect and diminished

Reason — Image formed by a convex mirror is always erect, virtual and diminished.

Question 4i

The image formed by a concave mirror is of the same size as the object, if the object is placed:

1. at the focus
2. between the pole and the focus
3. between the focus and the centre of curvature
4. at the centre of curvature

Answer

at the centre of curvature

Reason — If the object is placed at the centre of curvature of concave mirror the image formed is real, inverted and of the same size as that of object.

Question 4j

A convex mirror is used:

1. as a shaving mirror
2. as a head mirror by a dentist
3. as a rear view mirror by a driver
4. as a reflector in torch

Answer

as a rear view mirror by a driver

Reason — A convex mirror always forms erect and small images and this fact enables to use it as a rear view mirror so that the driver can see all the traffic behind him.

Short/Long Answer Questions

Question 1

State the speed of light in:

(a) air

(b) water and

(c) glass.

Answer

(a) The speed of light in air is 3 x 10⁸ m s^-1.

(b) The speed of light in water is 2.25 x 10⁸ m s^-1.

(c) The speed of light in glass is 2 x 10⁸ m s^-1.

Question 2

How does the speed of light determine the optical density of a medium?

Answer

A medium is said to be denser if the speed of light in it decreases, while it is said to be rarer if the speed of light in it increases.

Question 3

Which is optically denser: water or air? Give reason.

Answer

Water is optically denser than air because speed of light in water (2.25 x 10⁸ m s^-1) is less than speed of light in air (3 x 10⁸ m s^-1).

Question 4

Out of air and glass, which is optically rarer? Give reason.

Answer

Air is optically rarer than glass as speed of light is more in air (3 x 10⁸ m s^-1) than the speed of light in glass (2 x 10⁸ m s^-1).

Question 5

What do you understand by refraction of light?

Answer

The change in direction of path of light when it passes from one optically transparent medium to another is called refraction of light.

Question 6

Describe an experiment to show that a light ray bends when it passes from one transparent medium into another transparent medium.

Answer

Experiment to show that a light ray bends when it passes from one transparent medium into another transparent medium.

1. Take an empty beaker and a pencil. Place the pencil ABC obliquely in the beaker.
2. Look at the pencil from side. It appears straight as shown in figure (a).
3. Now pour water into the beaker up to its brim.

Pencil appears to be bent as ABD at the surface of water as shown in the figure (b).

The light ray coming from the tip C of the pencil bends at the surface of water as it enters in air and it appears to be coming from point D. Due to refraction of light from water to air, the pencil ABC appears as ABD.

This shows that when a ray of light passes from one transparent medium into another transparent medium it bends.

Question 7

Draw a ray diagram to show that the depth of a vessel containing water when seen from above appears to be less than its real depth.

Answer

The ray diagram showing the apparent depth of a vessel full of water is given below:

The real depth of water is AO but when viewed obliquely from above, its depth appears to be AI due to refraction of light from water to air.

Question 8

Define the following terms:

(a) Incident ray

(b) Refracted ray

(c) Angle of incidence

(d) Angle of refraction

Answer

(a) Incident ray — The ray of light falling on the surface separating the two media is called the incident ray.

(b) Refracted ray — The ray of light traveling in the other medium in the changed direction is called the refracted ray.

(c) Angle of incidence — The angle between the incident ray and the normal is called the angle of incidence (i).

(d) Angle of refraction — The angle between the refracted ray and the normal is called the angle of refraction (r).

Question 9

A ray of light falls normally on a glass slab. What is the angle of incidence?

Answer

When a ray of light falls normally on a glass slab the angle between incident ray and normal is 0°. So, the angle of incidence is 0°.

Question 10

A ray of light travels from a rarer medium to a denser medium. How will it bend?

Answer

When a ray of light travels from a rarer medium to a denser medium, it bends towards the normal because the speed of light decreases in denser medium.

Question 11

A ray of light travels from a denser medium to a rarer medium. How will it bend?

Answer

When a ray of light travels from a denser medium to a rarer medium, it bends away from the normal because the speed of light increases in the rarer medium.

Question 12

The diagram given below in Fig. 5.38 shows a ray of light AO falling on a surface separating two media. Draw the refracted ray in each, case.

(a)

(b)

(c)

Answer

(a) When light ray AO passes from air (rarer medium) to water (denser medium), it bends towards the normal as shown below.

(b) When light ray AO travels from water (denser medium) to air (rarer medium), it bends away from the normal as shown in the diagram below.

(c) When light ray AO is incident normally to the surface, it passes undeviated as shown in the figure below.

Question 13

Draw a diagram showing the refraction of a light ray from water to glass. Label on it the incident ray, the angle of incidence (i), and the angle of refraction (r).

Answer

Below figure shows the refraction of a light ray from water to glass:

AO — incident ray
i — angle of incidence
r — angle of refraction

Question 14

The diagram in the Fig. 5.39 shows a ray of light AO falling on a rectangular glass slab PQRS. Complete the diagram till the ray of light emerges out of the slab. Label on the diagram the incident ray, the refracted ray and the emergent ray.

Answer

The completed diagram showing the ray of light till it emerges out of the slab is given below :

AO — incident ray
OB — refracted ray
BC — emergent ray

Question 15(a)

Explain the following:

A coin placed at the bottom of a vessel appears to be raised when water is poured in the vessel.

Answer

The below ray diagram shows why a coin appears raised when placed in a vessel filled with water:

When water is poured in the vessel, the ray of light from point A of the coin, travelling in a straight line changes its direction (i.e. it bends) at the surface of water and reaches the eye. The ray now appears to come from point B instead of point A to our eye. So, the coin appears to be raised from position A to position B.

Question 15(b)

Explain the following:

A straight stick partly dipped in water obliquely, appears to be bent at the surface of water.

Answer

A straight stick partly dipped in water obliquely, appears to be bent at the surface of water due to refraction of light. The light rays coming from the tip C of the stick, at the surface of water undergoes refraction and bends away from the normal and appears to us as coming from point D (as shown in figure). Due to refraction of light from water to air the pencil ABC appears to be bent as ABD at the surface of water.

Question 15(c)

Explain the following:

The sun is seen before the sunrise and after the sunset.

Answer

The upper atmospheric layers are warmer than the layers near the surface of the earth before sunrise and after sunset, so the layers near the earth's surface are more denser than upper layers. When sun is below horizon the rays of sun passes from rarer to denser medium and suffers refraction by bending towards the normal. Due to continuous bending of light rays the sun appears to be above horizon. So the sun is seen before the sunrise and after the sunset.

Question 16

What is a mirage? Give a reason for its formation.

Answer

Sometimes, in deserts, an inverted image of a tree is seen which gives a false impression of water under the tree. This is called a mirage.

Mirage is formed due to the refraction of light. In a desert, during the day-time, the hot sand rapidly heats the layers of air in contact with it. As a result, the layers of air near the ground are rarer and the successive upper layers are denser than those below them.

The light rays from sun after reflection from the top of a tree travel from denser (cold upper layers of air) to rarer medium (warmer lower layer of air) and undergo refraction bending away from the normal. On successive refraction, the angle of refraction increases and the angle of incidence of ray going from denser to rarer medium also increases till a stage is reached when the angle of refraction becomes 90°. On further increase in angle of incidence the light ray is not refracted but is reflected. This reflected ray now travels from rarer to denser layer and bends towards the normal at each refraction. On reaching the eye of the observer, an inverted image of the tree is seen. Thus, it gives a false impression of a pool of water in front of the tree.

Question 17

What is a prism? Draw a ray diagram to show the refraction of a light ray through a prism.

Answer

A prism is a transparent medium bounded by five plane surfaces with a triangular cross section.

In symbol form it is represented by the triangle ABC :

The refraction of a light ray through a prism is shown below.

The light ray PQ of a single colour is incident obliquely on the surface AB of the prism. As the ray PQ moves from air (rarer) to glass (denser), it bends towards normal and travels as refracted ray QR. The refracted ray QR at the face AC of the prism again undergoes refraction (bends away from the normal) as it moves from glass (denser) to air (rarer) and emerges out as ray RS (emergent ray) from the prism.

Question 18

What do you mean by the term dispersion?

Answer

The splitting of white light into its constituent colours while passing through a prism is called dispersion of light.

Question 19

A ray of white light falls on a prism. Draw a ray diagram to show that the prism disperses the white light.

Answer

Dispersion of a white light into its constituent colours by a prism is shown in the below figure.

Question 20

In figure, AO is the ray of white light falling on a prism PQR. Complete the diagram till the light emerges out from the prism and falls on the screen.

Answer

White light splits up into seven constituent colours after passing through a prism. These seven colours VIBGYOR arranged from bottom to top are seen on the screen.

Question 21

What do you understand by the term spectrum? Name the various colours present in the spectrum of sunlight.

Answer

Spectrum is the coloured band obtained on a screen on passing the white light through a prism. The various colours present in the spectrum of sunlight are violet, indigo, blue, green, yellow, orange and red.

Question 22

You are given a disc divided into seven sectors with colours violet, indigo, blue, green, yellow, orange and red in them. What would be its color when it is rotated rapidly?

Answer

The disc appears white when rotated rapidly. The reason is that seven colours violet, indigo, blue, green, yellow, orange and red are the constituent colours of white light. When these colours are combined they produce the white colour effect.

Question 23

State the two laws of reflection of light.

Answer

The two laws of reflection of light are:

1. The angle of incidence i is equal to the angle of reflection r.
2. The incident ray, the reflected ray and the normal, all lie in the same plane.

Question 24

What is a spherical mirror?

Answer

A spherical mirror is made by silvering a part of a hollow glass sphere. The surface on which silvering is done is called the silvered surface and the reflection of light takes place from the other surface known as the reflecting surface.

Question 25

State the two kinds of spherical mirrors and distinguish them with the aid of proper diagrams.

Answer

The two kinds of spherical mirrors are:

1. Concave mirror
2. Convex mirror

Figure of concave and convex mirror is shown below:

Question 26

Explain the following terms:

(a) Pole

(b) Centre of curvature

(c) Radius of curvature

(d) Principal axis

Show them on separate diagrams for each of the concave and convex mirrors.

Answer

(a) Pole — The geometric centre of the spherical surface of the mirror is called the pole of the mirror. It is represented by the symbol P.

(b) Centre of curvature — The centre of curvature of a mirror is the centre of the sphere of which the mirror is a part. It is represented by the symbol C.

(c) Radius of curvature — The radius of curvature of a mirror is the radius of the sphere of which the mirror is a part. It is represented by symbol R.

(d) Principal axis — It is a straight line joining the pole of the mirror to its centre of curvature.

Question 27

What do you understand by the focus and focal length of a spherical mirror? Show them on separate diagrams for each of a concave mirror and a convex mirror.

Answer

The focus (F) of a concave mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, meet (converge) after reflection from the mirror.

The focus (F) of a convex mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, appear to meet after reflection from the mirror.

The distance of the focus from the pole of the mirror is called the focal length of the mirror.

Question 28

Draw suitable diagrams to illustrate how a beam of light incident parallel to the principal axis is reflected by:

(a) a concave mirror and

(b) a convex mirror.

Answer

(a) A beam of light incident parallel to the principal axis of a concave mirror, after reflection meets at a point called focus of the mirror.

(b) A beam of light incident parallel to the principal axis of a convex mirror, after reflection appears to meet at a point called focus of the convex mirror.

Question 29

How is a spherical mirror used to converge a beam of light at a point? Name the type of mirror used.

Answer

The mirror used is a concave mirror. The beam of light is incident parallel to the principal axis of the concave mirror. After reflection, the beam converges to a point called the focus of the concave mirror.

Question 30

How is a spherical mirror used to diverge a beam of light from a point? Name the type of mirror used.

Answer

The mirror used is a convex mirror. The beam of light is incident parallel to the principal axis of the convex mirror. After reflection, the beam of light diverges and appear to meet at a point called the focus of the convex mirror.

Question 31

State the direction of incident ray which after reflection from a spherical mirror gets reflected along its own path. Give a reason.

Answer

A ray passing through the centre of curvature of a spherical mirror is reflected along its own path.
A line joining the centre of curvature to any point on the surface of mirror is always normal to it. Thus, a ray passing through the centre of curvature is incident normally on the spherical mirror. Its angle of incidence is zero, therefore, the angle of reflection is also zero. It means that the ray gets reflected along its own path.

Question 32

How is the focal length of a spherical mirror related to its radius of curvature?

Answer

The focal length of a spherical mirror is half of its radius of curvature.

Focal length = $\dfrac{\text{1}}{\text{2}}$ x Radius of curvature

Question 33

The diagrams (Fig. 5.41) given below show two parallel rays 1 and 2 incident on:

(a) a concave mirror,

(b) a convex mirror.

Draw the reflected rays and mark the focus by the symbol F.

Answer

The completed diagrams with focus marked are shown below :

C — Centre of curvature,
F — Focus,
P — Pole

Question 34

Complete the following diagrams in Fig. 5.42 by drawing the reflected rays for the incident rays 1 and 2 if F is the focus and C is the centre of curvature.

Answer

Ray 1 passing through the focus of a concave mirror gets reflected parallel to the principal axis.

Ray 2 passing through the centre of curvature of a concave mirror is reflected along its own path.

Ray 1 appearing to pass through the focus of a convex mirror gets reflected parallel to the principal axis.

Ray 2 directed in the direction of centre of curvature of a convex mirror is reflected along its own path.

Question 35

Which are the two convenient rays that are chosen to construct the image formed by a spherical mirror for a given object? Explain with the help of suitable ray diagrams.

Answer

The two convenient rays that are chosen to construct the image formed by a spherical mirror for a given object are:

(i) A ray that passes through the center of curvature — A line joining the centre of curvature to any point on the surface of mirror is normal to the mirror at that point, therefore a ray AD passing through the center of curvature C (or appearing to pass through through the centre of curvature C) is incident normally on the spherical mirror.
Since it's angle of incidence is zero, therefore the angle of reflection will also be zero and the ray AD gets reflected along it's own path DA as shown below:

(ii) A ray parallel to the principal axis — A ray of light AD incident parallel to the principal axis, after reflection passes either through the focus F(in a concave mirror) or will appear to come from the focus F (in a convex mirror) along DB as shown below:

Question 36

Draw a ray diagram to show the formation of the image of an object placed beyond the centre of curvature of a concave mirror. State the position, size and nature of the image.

Answer

Nature — Image is real and inverted.

Position — The image is formed between the centre of curvature and focus of the concave mirror.

Size — The size of the image is smaller than that of the object.

Question 37

Draw a ray diagram to show the formation of the image of an object placed at the centre of curvature of a concave mirror. State the position, size and nature of the image.

Answer

Nature — Image is real and inverted.

Position — The image is formed at the centre of curvature of the concave mirror.

Size — The size of image and object is the same.

Question 38

Draw a ray diagram to show the formation of image of an object placed between the focus and centre of curvature of a concave mirror. State the position, size and nature of the image.

Answer

Nature — Image is real and inverted.

Position — The image is formed beyond centre of curvature of the mirror.

Size — The size of image is bigger than that of object.

Question 39

Draw a ray diagram to show the formation of image of an object placed between the pole and focus of a concave mirror. State the position, size and nature of the image.

Answer

Nature — The image is virtual and erect.

Position — The image is formed behind the mirror.

Size — The size of image is bigger than that of object.

Question 40

Draw a ray diagram to show the formation of the image of an object placed on the principal axis of a convex mirror. State the position, size and nature of the image. What happens to the image as the object is moved away from the mirror?

Answer

Nature — Image is virtual and erect.

Position — The image is formed behind the mirror between focus and pole of the convex mirror.

Size — Image is diminished in size.

As the object is moved away from the mirror, the image moves towards the focus and the size decreases to a point.

Question 41

Draw separate diagrams for the formation of virtual image of an object by

(a) a concave mirror and

(b) a convex mirror.

State the difference of the two images.

Answer

(a) When an object is placed between the pole and focus of a concave mirror, the image is formed behind the mirror. A virtual, erect and enlarged image of the object is formed.

(b) In convex mirror for the object placed at finite distance, the image formed is always virtual, erect, diminished and it is formed between pole and focus of the mirror.

Question 42

Name the mirror which always forms an erect and virtual image. What is the size of the image as compared to that of the object?

Answer

Convex mirror always forms an erect and virtual image. The image size is always smaller as compared to that of the object.

Question 43

Name the mirror which forms an erect, virtual and enlarged image of an object. What is the position of object relative to the mirror?

Answer

Concave mirror forms an erect, virtual and enlarged image of an object. The object should be placed between the pole and focus of the concave mirror.

Question 44

What is a real image? Name the mirror which can be used to obtain the real image of an object. What should be the position of the object relative to the mirror?

Answer

A real image is formed when the reflected rays actually meet at a point. It can be obtained on a screen.

Concave mirror is used to obtain the real image of an object. If the object is located at any position except between pole and focus of the concave mirror than the image formed is real.

Question 45

How can a concave mirror be used to obtain a virtual image of an object? Draw a diagram to illustrate your answer.

Answer

The object should be placed between the pole and focus of the concave mirror to obtain a virtual image of the object.

Question 46

State two uses of a concave mirror.

Answer

The two uses of a concave mirror are:

1. It is used as a shaving mirror.
2. It is used as a doctor's head mirror.

Question 47

State two uses of a convex mirror.

Answer

The two uses of a convex mirror are:

1. It is used as a rear view mirror.
2. It is used as a reflector in street lamps.

Question 48

A driver uses a convex mirror as a rear view mirror. Explain the reason with the help of a ray diagram.

Answer

A convex mirror diverges the incident light rays and always form a small, erect image between its pole and focus. This enables the driver to use it as a rear view mirror so that he could able to see all the traffic behind him approaching the mirror. It has wider field of view than a plane mirror.

Question 49

State the kind of mirror used:

(a) by a dentist, and

(b) as a street light reflector.

Answer

(a) Concave mirror is used by a dentist because if a parallel beam of light is incident on a concave mirror, it converges the beam to the focus. This enables the dentist to concentrate light on small area to be examined like teeth.

(b) A convex mirror is used as a street light reflector because it diverges the light rays incident on it.

Question 50

Name the kind of mirror used to obtain:

(a) a real and enlarged image,

(b) a virtual and enlarged image,

(c) a real and diminished image, and

(d) a virtual and diminished image.

Answer

(a) Concave mirror is used to obtain a real and enlarged image when object is placed between its centre of curvature and focus.

(b) Concave mirror is used to obtain a virtual and enlarged image when object is placed between its focus and pole.

(c) Concave mirror is used to obtain a real and diminished image when object is placed beyond its centre of curvature.

(d) Convex mirror produces a virtual and diminished image.