Living Science Physics Class 8 ICSE Solutions Chapter 5 Light Energy

ICSE Living Science Physics for Class 8 Solutions Chapter 5 Light Energy

Check Your Progress

A. Write true or false. Correct the false statements.
Question 1.
A concave mirror is also known as a diverging mirror.
Answer:
False.
Correct : A concave mirror is also known as a converging mirror.

Question 2.
The focal length of a spherical mirror is twice the radius of curvature.
Answer:
False.
Correct : The focal length of a spherical mirror is half the radius of curvature.

Question 3.
While sketching ray diagrams, the object should always be kept in front of the polished surface.
Answer:
False.
Correct: While sketching ray diagrams, the object should always be kept in front of the reflecting surface.

Question 4.
A ray of light incident at the pole of a spherical mirror is reflected along a path, making the same angle with the principal axis, as the incident ray does.
Answer:
True

Question 5.
A ray of .light parallel to the principal axis, after reflection from a concave mirror, will pass through the centre of curvature.
Answer:
False.
Correct : A ray of light parallel to the principal axis, after reflection from a concave mirror, will pass through the its focus.

B. Fill in the blanks.

1. A …………. image can be obtained on a screen.
2. The ………… of a mirror denotes its size.
3. The length of the straight line joining two opposite extreme points on the periphery of a spherical mirror is called …………..
4. The center of the hollow sphere of which a spherical mirror forms a part is called its …………
5. A virtual image is always ………….
6. In a concave mirror, an incident ray passing through the focus is reflected parallel to the ………..

Answer:

1.  A real image can be obtained on a screen.
2. The aperture of a mirror denotes its size.
3. The length of the straight line joining two opposite extreme points on the periphery of a spherical mirror is called focus.
4. The center of the hollow sphere of which a spherical mirror forms a part is called its linear aperture.
5. A virtual image is always inverted.
6. In a concave mirror, an incident ray passing through the focus is reflected parallel to the principal axis.

Think and answer

Question 1.
Why can concave mirrors not be used as rear view mirrors in vehicles ?
Answer:
Concave mirror cannot be used as rear view mirrors in vehicles. This is because if they are used, they will mostly form a real and inverted image of the vehicles behind. Moreover, the size of the image of the vehicles behind will also vary as the distance of the vehicles behind changes from the rear view mirror. As the vehicles behind
approach the rear view mirror, their images will keep on magnifying, limiting the field of view of the vehicles coming behind.

Think and answer

Question 1.
A ray of light travelling in medium A (µ = 1) enters medium B (µ = 1.4).
1. Will the ray of light bend towards or away from the normal ?
2. In which medium will the speed of light be more ?
Answer:
As medium B is optically denser than medium A, therefore, the ray will bend towards the normal while travelling from medium A to medium B. Speed of light will be more in medium, A.

Check Your Progress

Answer the following :
Question 1.
Define refraction. State the two laws of refraction.
Answer:
The phenomenon of bending of light is known as refraction. The two laws of refraction are given as follows.
a. The incident ray, the normal at the point of incidence and the refracted ray all lie on the same plane.
b. For a given pair of media of different densities, the ratio of the sine of angle of incidence to the sine of angle of refraction is always a constant, that is,

Question 2.
What is the speed of light in vacuum ?
Answer:
The speed of light in vacuum is 3 x 10⁸ m/s,

Question 3.
Define refractive index.
Answer:
Refractive index of a medium with respect to vacuum is defined as the ratio of the speed of light in vacuum to the speed of light in the given medium.

Question 4.
Give two examples of refraction of light.
Answer:
Two examples of reflection of light are :
a. A pencil placed in water appears to be bent at the point it enters the water.
b. A coin immersed in water appears to be raised.

Question 5.
What do you understand by lateral displacement.
Answer:
When a ray of light passes through a rectangular glass slab, it emerges parallel to the incident ray and is shifted from its original path. The perpendicular distance between the emergent ray and the incident ray is called the lateral displacement.

A. Tick the most appropriate answer.

Question 1.
A mirror in which the inner hollow surface is the reflecting surface is called a
a. concave mirror.
b. convex mirror.
c. plane mirror.
d. hollow mirror.
Answer:
a. concave mirror.

Question 2.
The geometrical centre of a spherical mirror is called the
a. centre of curvature
b. pole of the mirror
c. principal focus.
d. focal length.
Answer:
b. pole of the mirror.

Question 3.
Which of the following relations is true for spherical mirrors?
a. f = r
b. f = 2r
c. r = 2f
d. 2f = 3r
Answer:
c. r = 2f

Question 4.
When an object is placed at the principal focus of a concave mirror, the image will be formed at the
a. focus.
b. centre of curvature,
c. pole.
d. infinity.
Answer:
d. infinity.

Question 5.
The image formed when reflected rays only appear to meet but do not actually meet is
a. real.
b. virtual,
c. inverted.
d. cannot say
Answer:
b. virtual.

Question 6.
If the object is in front of a convex mirror, then the image is always
a. real.
b. inverted,
c. diminished.
d. enlarged.
Answer:
c. diminished.

Question 7.
Which of the following mirrors is used by dentists?
a. concave
b. convex
c. plane
d. conical
Answer:
a. concave

Question 8.
When a ray of light travels from a rarer medium to a denser medium, it
a. bends towards the normal.
b. bends away from the normal.
c. does not deviate from its path.
d. travels along the normal.
Answer:
b. bends away from the normal.

Question 9.
The number of constituent colours of white light is
a. 5.
b. 3.
c. 7.
d. 9.
Answer:
c. 7.

Question 10.
The phenomenon of light responsible for the formation of rainbow is –
a. reflection.
b. refraction,
c. dispersion.
d. lateral inversion.
Answer:
c. dispersion.

B. Fill in the blanks.

1. A spherical mirror with the outer surface as the reflecting surface is called a ………….. mirror.
2. The radius of curvature of a spherical mirror is always ………….. its focal length.
3. A ray of light passing through the , retraces its path.
4. A ………… mirror is used as a shaving mirror.
5. A convex mirror never produces …………. images.
6. The phenomenon of bending of light is called …………. of light.
7. In refraction, the incident ray, refracted ray and the ……….. normal lie on the same
8. If a ray of light strikes the surface of separation at …………. degrees, the ray goes undeviated.
9. The phenomenon of splitting of white light into its seven constituent colours after passing through a prism is called …………….
10. In a spectrum …………. colour bends the most …………. whereas colour bends the least.

Answer:

1. A spherical mirror with the outer surface as the reflecting surface is called a convex mirror.
2. The radius of curvature of a spherical mirror is always twice its focal length.
3. A ray of light passing through the centre of curvature retraces its path.
4. A concave mirror is used as a shaving mirror.
5. A convex mirror never produces real images.
6. The phenomenon of bending of light is called refraction of light.
7. In refraction, the incident ray, refracted ray and the normal lie on the same plane.
8.  If a ray of light strikes the surface of separation at 90° degrees, the ray goes undeviated.
9. The phenomenon of splitting of white light into its seven constituent colours after passing through a prism is called dispersion.
10. In a spectrum violet colour bends the most, whereas red colour bends the least.

C. Write true or false. Correct the false statements.

Question 1.
A spherical mirror is a part of a hollow glass sphere that is polished on one side.
Answer:
True

Question 2.
The geometrical centre of a spherical mirror is called its pole.
Answer:
True

Question 3.
An incident ray passing through the focus of a convex mirror goes undeviated after reflection.
Answer:
False.
Correct : An incident ray passing through the centre of curvature goes undeviated after reflection.

Question 4.
A concave mirror is used in solar heaters.
Answer:
True

Question 5.
A real image is always erect.
Answer:
False.
Correct : A real image is always inverted.

Question 6.
Light travels faster in air than in water.
Answer:
True

Question 7.
The ray of light emerging out of a glass slab is always parallel to the incident ray.
Answer:
True

Question 8.
A prism is a glass block that has a rectangular cross section, formed by three triangular faces.
Answer:
False.
Correct : A prism is a glass block that has a triangular cross section, formed by three rectangular faces.

Question 9.
The angle through which the incident ray is deviated by the prism is called the angle of deviation.
Answer:
True

Question 10.
A rainbow is formed because water droplets of the atmosphere behave like glass slabs.
Answer:
False.
Correct : A rainbow is formed because water droplets of the atmosphere behave like tiny prisms.

D. Match the columns.

Position Of The Object In Front Of A Concave Mirror	Nature Of The Image
1. At infinity	a. real, inverted and diminished
2. Between F and P	b. real, inverted and enlarged
3. Beyond C	c. real, inverted and highly diminished
4. Between C	d. real, inverted and same size as the object
5. At C	e. virtual, erect and enlarged

Answer:

Position Of The Object In Front Of A Concave Mirror	Nature Of The Image
1. At infinity	c. real, inverted and highly diminished
2. Between F and P	e. virtual, erect and enlarged
3. Beyond C	a. real, inverted and diminished
4. Between C	b. real, inverted and enlarged
5. At C	d. real, inverted and same size as the object

E. Define the following.

1. Centre of curvature
2. Pole
3. Aperture
4. Principal axis
5. Focal length
6. Angle of incidence
7. Angle of refraction
8. Refractive index
9. Snell’s law of refraction
10. Angle of emergence
11. Angle of deviation of the prism

Answer:

1. Centre of curvature – The geometrical centre of the hollow sphere of which the spherical mirror is a part is called its centre of curvature.
2. Pole – The geometrical centre of the spherical mirror is called the pole of the mirror.
3. Aperture – The area of the mirror from which reflection takes place is called its aperture.
4. Principal axis – The straight line passing through the pole and the centre of curvature of the spherical mirror is called the principal axis.
5. Focal length – The distance between the pole and the principal focus is called the focal length of the spherical mirror.
6. Angle of incidence – The angle between the incident ray and the normal is called the angle of incidence.
7. Angle of refraction – The angle between the refractec ray and the normal is called the angle of refraction.
8. Refractive index – Refractive index of a medium with respect to vacuum is defined as the ratio of the speed of light in vacuum to the speed of light in the given medium.
9. Snell’s law of refraction – It states that for a given pair of media of different densities, the ratio of the sine of angle of incidence to the sine of angle of refraction is always a constant.
10. Angle of emergence – The angle between the emergent ray arid the normal is called angle of emergence.
11. Angle of deviation of the prism – The angle through which the incident ray is deviated by the prism is called the angle of deviation of the prism.

F. Answer the following in short.

Question 1.
The radius of curvature of a concave mirror is 16 cm. What is its focal length?
Answer:
Radius of curvature of a concave mirror, r = 16 cm
Focal length of the mirror, f = \(\frac {r}{2}\)= \(\frac {16 cm}{ 2 }\) = 8 cm

Question 2.
The distance between the pole and the principal focus of a spherical mirror is 10 cm. What is the focal length of the mirror?
Answer:
Distance between the pole and the principal focus of a spherical mirror = 10 cm
Focal length of a spherical mirror = distance between the pole and the principal focus 10cm

Question 3.
The focal length of a spherical mirror is 8 cm. What is its radius of curvature?
Answer:
Focal length of a spherical mirror,f = 8cm
Radius of curvature of the spherical mirror, r = 2f
= 2 x 8 cm = 16 cm

Question 4.
State any three uses of concave mirrors.
Answer:
Some of the uses of concave mirrors are listed below.
a. They are used in solar heaters to concentrate sun rays towards the food ¡n the container to be heated.
b. They are used by dentists to see enlarged image of tooth and other area in mouth.
c. They are used as shaving mirrors.

Question 5.
Can a convex mirror form an ¡mage larger than the object?
Answer:
No. a convex mirror always form an image smaller than the object.

Question 6.
State the laws of refraction.
Answer:
The two laws of refraction are given as follows.
a. The incident ray, the normal at the point of incidence and the refracted ray all lie on the same plane.
b. For a given pair of media of different densities, the ratio of the sine of angle of incidence to the sine of angle of
refraction is always a constant, that is,

Question 7.
How does the refractive index of a transparent medium affect the speed of light passing through it?
Answer:
More the refractive index of a transparent medium, less is the speed of light passing through it.

Question 8.
What is a spectrum? State the colours in the spectrum of sunlight obtained from a prism in their correct order,
Answer:
The band of seven component colours of white light obtained after dispersion is known as spectrum. The order of the seven colours is violet, indigo, blue, green, yellow, orange and red (VIBGYOR).

Question 9.
What happens when light is allowed to pass through two prisms placed next to each other such that their adjacent faces are parallel?
Answer:
When light is allowed to pass through two prisms placed next to each other such that their adjacent faces are parallel, the first prism disperses the light into its seven colours and the second prism re-unites all the seven components of light and a ray of white light emerges out from the prism.

Question 10.
What is meant by the refractive index of a medium with respect to air?
Answer:
Refractive index of a medium with respect to air is defined as the ratio of the speed of light in air to the speed of light in the given medium.

G. Answer the following in detail.

Question 1.
What do you mean by a spherical mirror? Distinguish between a concave mirror and a convex mirror. .
Answer:
A spherical mirror is a part of a hollow glass sphere with one of its curved surfaces polished in silver while the

Concave mirror :

• It is a spherical mirror whose inner surface (curving inwards) is the reflecting surface and the outer surface (bulging outwards) is polished.
• When a parallel beam of light falls on this mirror, it converges at a point after reflection.
• It is also known as converging mirror.
• It can form real or virtual image depending on the position of the object with respect to the mirror.

Convex mirror :

• It is a spherical mirror whose outer surface is the reflecting surface and the inner surface is polished.
  When a parallel beam of light falls on this mirror,
• it appears diverge from a point after reflection.
• It is also known as diverging mirror.
• It always form a virtual, erect and diminished image of the object irrespective of the position of the object with respect to the mirror.

Question 2.
State the rules for drawing ray diagrams for spherical mirrors. Support them with ray diagrams.
Answer:
In order to obtain the image of an object by drawing ray diagrams for spherical mirrors, any two of the following rays can be taken as the incident rays for locating the image.
a. An incident ray passing through the centre of curvature :
An incident ray passing through the centre of curvature of a concave mirror reflects back along the same path, without any deviation. In case of a convex mirror, an incident ray which appears to move towards the direction of centre of curvature is reflected back along the same path without any deviation.

b. An incident ray parallel to the principal axis :
In case of a concave mirror, an incident ray parallel to the principal axis passes through its focus after reflection. In case of a convex mirror, the incident ray after reflection appears to come from the focus of the mirror.

c. An incident ray passing through the focus :
In a concave mirror, an incident ray passing through the focus is reflected parallel to the principal axis. In case of a convex mirror, the incident ray, which appears to move in the direction of the focus of the mirror, is reflected parallel to the principal axis.

d. A ray of light incident at the pole of the mirror :
In a concave mirror, a ray of light incident at the pole gets reflected along a path such that the angle of incidence is equal to the angle of reflection. This is also true for a convex mirror.

Question 3.
Name the type of a spherical mirror which always forms a virtual, erect and diminished image. What is the position of such type of images with respect to the mirror? Draw a ray diagram for the same.
Answer:
Convex mirror always form a virtual, erect and diminished image. The image is always formed between the focus and the pole of the mirror, irrespective of the position of the object in front of the mirror.

Question 4.
Why does a pencil appear to be bent when placed in a beaker of water?
Answer:
A pencil appears to be bent when placed in a beaker of water due to refraction of light. The ray of light from the tip of the pencil (A) travels from an optically denser medium (water) to an optically rarer medium (air). The ray is refracted at the surface of water, and bends away from the normal.

When this ray reaches the eye of the viewer, it appears to come from a point A’. Similarly, all points between AB appear to come from corresponding points between A’B. Therefore, the pencil appears to be bent and shorter, when viewed from the top.

Question 5.
How is a rainbow formed^
Answer:
Rainbow is formed due to dispersion of sunlight and is visible in the sky after a spell of rain. Just after a rainfall,
there are a large number of water droplets suspended in the atmosphere. These droplets act like tiny prisms. When sunlight passes through these spherical droplets, it undergoes dispersion. With a large number of water droplets contributing to this phenomenon, we get to see a huge continuous band of seven colours displayed in the sky.

Question 6.
A ray of light is incident at an angle i on the surface of a glass slab. If its angle of refraction inside the glass block is r, then find the relation between i and r assuming that light enters from air and emerges into air.
Answer:
The relation between i and r is given by the Snell’s law, which states that for a given pair of media of different densities, the ratio of the sine of angle of incidence to the sine of angle of refraction is always a constant, that is,

The value of the constant is fixed for a set of two transparent media and depends upon their nature. It is called the refractive index of the second medium with respect to the first. In case of a glass slab, light ray is entering the glass slab from the air. So, the ratio of sin i to sin r is equal to the refractive index of glass with respect to air.
= refractive index of glass with respect to air

Question 7.
Describe dispersion of white light. Why do different colours of light bend to different extents when passed through a prism?
Answer:
The phenomenon of splitting of white light into its constituent colours is called dispersion. When white light is allowed to pass through a prism, it splits into a band of seven colours-violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Dispersion of light occurs because different colours of light bend to different extents when passed through a prism.

This is because the refractive index of a medium with respect to another medium is different for different colours. Red colour deviates (or bends) the least, whereas violet deviates the most. This is why red forms the top band of the spectrum and violet forms the bottom band.

Question 8.
Explain why two prisms placed next to each other such that their adjacent faces are parallel, behave like a glass slab.
Answer:
If two prisms are placed next to each other such that their adjacent faces are parallel to each other and a ray of white light is allowed to fall on the refracting surface of one of them, then the first prism on which the ray of light falls will disperse the light into its seven colours. When this dispersed light enters the second prism, all the seven components of light re-unite and a ray of white light emerges out from the prism. Therefore, two prisms kept opposite to each other behave like a glass slab.

H. Give reasons for the following.

Question 1.
Concave mirrors are also known as converging mirrors,
Answer:
In a concave mirror, all rays of light parallel to the principal axis converge after reflection and meet at the principal focus. Due to this property, concave mirrors are also known as converging mirrors.

Question 2.
Concave mirrors are used by dentists.
Answer:
Concave mirrors are used by dentists to see enlarged image of tooth and other area in mouth because concave mirrors produce virtual and magnified images of close objects. .

Question 3.
Convex mirrors are preferred over plane mirrors as rear view mirrors in cars.
Answer:
Convex mirrors are preferred over plane mirrors as rear view mirrors in cars because convex mirrors always form an erect and diminished image, which gives a wider view of the vehicles coming behind.

Question 4.
Stars twinkle in the sky.
Answer:
Stars appear to twinkle due to the refraction of light coming from the star from different layers of the earth’s atmosphere.

Question 5.
The coin immersed in water appears to be raised.
Answer:
The coin immersed in water appears to be raised due to refraction of light at the surface separating air and water.

I. Solve the following numerical problems.

Question 1.
If the speed of light in glass is 2 x 10⁸ m/s, and that in air is 3 x 10⁸ m/s, calculate the refractive index of glass with respect to air.
Answer:
Speed of light in glass = 2 x 10⁸ m/s
Speed of light in air = 3 x 10⁸ m/s
Refractive index of glass with respect to air

= 1.5

Question 2.
The speed of light in a transparent medium is 2.44 x 10⁸m/s. Find the refractive index of the medium.
Answer:
Speed of light in the transparent medium = 2.44 x 10⁸ m/s
Speed of light in vacuum = 3 x 10⁸ m/s
Refractive index of the medium with respect to vacuum

speed of light in the medium 2.44 x 108 m/s
= 1.23

Question 3.
Find the speed of light in a transparent medium if the refractive index of the medium is 1.8.
Answer:
Refractive index of a medium = 1.8
Speed of light in vacuum = 3 x 10⁸ m/s
From the expression,
Refractive index of the medium with respect to vacuum

we get,

Question 4.
Find the refractive index of a medium if the speed of light in that medium is 2.25 x 10s m/s.
Answer:
Speed of light in the medium = 2.25 x 10⁸ m/s
Speed of light in vacuum = 3 x 1o⁸ m/s
Refractive index of the medium with respect to vacuum

Question 5.
The speed of light in diamond is found to be 1,24,000 km/s. What is the refractive index of diamond?
Answer:
Speed of light in diamond = 1,24,000 km/s
= 1.2 x 10⁵ km/s = 1.2 x 10⁸ m/s
Speed of light in vacuum = 3 x 108 m/s
Refractive index of diamond with respect to vacuum

J. Draw ray diagrams of the following.

Question 1.
Refraction of light through:
a. a parallel-sided glass slab
b. a prism
Answer:
(a)

(b)

Question 2.
Behaviour of a ray of light passing through two prisms placed next to each other such that their adjacent faces are parallel.
Answer:

Question 3.
Image formed by a concave mirror if the object is placed :
a. at infinity
b. between C and F
c. at F
d. between F and P
e. at C
Answer:
a. at infinity

b. between C and F

c. at F

d. between F and P

e. at C

Think and answer

Question 1.
There is a coloured stone at the bottom of a swimming pool. Jasvinder is trying to hit it with an arrow. However, each time he misses the target by a narrow margin. Can you explain to Jasvinder why he misses the target and how can he hit it with more accuracy?
Answer:
The coloured stone when viewed from the top 01 the swimming pool appears to be raised due to refraction of light. Jasvinder is actually hitting the coloured stone where it appears to be and hence misses the target whenever he tries to hit it with an arrow. If he hits the target by aiming at a position a little below the position of the stone visible from the top, then he might be able to hit the target.

Question 2.
An object is placed at the centre of curvature of a concave mirror of radius of curvature 20 cm and focal length 10 cm. At what distance from the pole will its image be formed?
Answer:
Radius of curvature of a concave mirror 20 cm Focal length of the mirror = 10 cm If the object is placed at the centre of curvature, then the image will also be formed at the centre of the curvature. So, the image will be formed at a distance of 20 cm from the pole of the mirror.

Question 3.
Why does a paper catch fire if a concave mirror is used to focus sunlight?
Answer:
A paper catches fire if a concave mirror is used to focus sunlight because concave mirror converges the parallel beam of light from the sun onto the paper. The large amount of heat getting concentrated on the paper raises its temperature, causing it to bum.

Question 4.
Will it be easy to watch the activities of customers in’ a departmental store with concave mirrors? Why or why not?
Answer:
No, it won’t be easy to watch the activities of customers in a departmental store with concave mirrors as the image formed by them changes its nature and position depending on the position of the customers. The image increases in size if the customer approaches the mirror. So, the shopkeeper doesn’t get a wide field of view. Moreover, the image formed is generally real and inverted. So, it is not easy to track the activities of the customers with the help of concave mirrors.

Question 5.
Why do objects waver when seen on a really hot day?
Answer:
Objects waver on a really hot day due to the refraction of light by the different layers of the atmosphere.

Question 6.
Identify the concave and the convex reflecting surfaces among the two mirrors shown in the figure if both of them have equal focal lengths.
Answer:
Mirror 1 – Concave mirror
Mirror 2 – Convex mirror

Question 7.
List two essential conditions to observe a rainbow.
Answer:
Two essential conditions to observe a rainbow are :
1. Sun must be present in the sky behind the back of the observer.
2. Water droplets should be present in the atmosphere.

Living Science Physics Class 8 ICSE Solutions