# Chapter Unit - 2 : Light[Refraction and Total internal Reflection of Light] - Frank Solutions for Class 10 Physics ICSE

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## Chapter 2 - Light[Refraction and Total internal Reflection of Light] Excercise 102

Question 1
What do you mean by the term refraction?
Solution 1
Refraction is the bending of light at the surface of separation, which takes when it passes from one optical medium to another optical medium with different optical densities.
Question 2
State Snell's law.
Solution 2
Question 3
Does reflection also take place with the refraction?
Solution 3
Yes, reflection also takes place with the refraction.
Question 4
What is the cause of refraction of light?
Solution 4
When a ray of light passes from one medium to another, its direction (except for normal incidence) changes because of change in speed of light. Thus, refraction occurs because light travels with different speed in different media.
Question 5
A ray of light is incident normally on a plane glass slab. What will be the angle of refraction and the angle of deviation for the ray?
Solution 5
The ray of light which is incident normally on the surface separating the two media passes undeviated. Such ray suffers no bending, thus, angle of refraction and angle of deviation is 0o.
Question 6
Name the phenomenon which enables an empty test tube placed inside water shine like a mirror.
Solution 6
The phenomenon is total internal reflection
Question 7
Write the SI unit for refractive index.
Solution 7
Refractive index being the ratio of similar quantities has no units.
Question 8
What do you mean by the statement that the refractive index of diamond is 2.42?
Solution 8

Question 9
Plot a graph between
(i)    Angle of incidence versus angle of reflection,
(ii)    Sine of angle of incidence versus sine of angle of refraction,
(iii)    Angle of incidence versus angle of refraction.
Solution 9
Question 10
How is the refractive index of a medium related to the speed of light in it?
Solution 10
Question 11
For which colour of white light, is the refractive index of a transparent medium the least?
Solution 11
Red colour of white light has least refractive index.
Question 12
For which colour of white light, is the refractive index of glass the most?
Solution 12
Violet colour of white light has highest refractive index.
Question 13
State the principle of reversibility of light.
Solution 13
According to the principle of reversibility, the path of a light ray is reversible.
For e.g. if light travels from air to water along a certain path, then if the path is reversed while travelling from water to air, it will follow exactly the same path.
Question 14
.
index of air with respect to water
Solution 14
Question 15
Define absolute refractive index of a medium.
Solution 15
The absolute refractive index of a medium is defined as the ratio of the speed of e-m radiation in free space to the speed of radiation in that medium.
Question 16
Define the term refractive index of a medium. What do you understand by the statement 'the refractive index of glass is 1.5 for white light'?
Solution 16
When light travels from one medium (air) to another medium (glass), it bends towards the normal. The extent of bending of light depends upon the speed of light (v2) in the second medium, compared to the speed of light (v1) in the first medium. The refractive index of the second medium w.r.t. the first medium (n21) is given by
n21 = Speed of light (v1) in first medium / speed of light (v2) in second medium
The refractive index of glass is typically around 1.5, meaning that light in glass travels at
c / 1.5 = 200,000 km/s. A low value of refractive index also indicates a large critical angle at the glass-air interface.
Question 17
Is dispersion same as deviation?
Solution 17
No, dispersion is not same as deviation.
Question 18
Define angle of deviation.
Solution 18
Angle of deviation may be defined as the angle between original path of incident ray and the path of refracted ray.
Question 19
Name the factors on which the deviation produced by a prism depends.
Solution 19
The value of angle of deviation produced by a prism depends upon:
(i) The angle of incidence
(ii) The material of prism
(iii) The angle of prism
(iv) The colour of wavelength of light used.
Question 20
Which of the following has the highest refractive index:
(a)        Glass        (b)    Water
(c)        Diamond    (d)    Ruby
Solution 20
(c) Diamond
Question 21
If a light ray does not undergo, refraction at the boundary between two media, the angle of incidence is:
(a)        00        (b)    450
(c)        600       (d)    900
Solution 21
(a) 0o
Question 22

A ray of light passes from air to water. In fig. 39, which of the ray A, B, C and D is the correct refracted ray?

Solution 22

Ray B is the correct refracted ray because a ray of light travelling from air (rarer medium) to water (denser medium) will bend towards the normal.

Question 23

In the fig. 40, name the ray which represents the correct path of light while passing through a glass block.

Solution 23

The correct path is that of ray 'B'.

## Chapter Unit - 2 - Light[Refraction and Total internal Reflection of Light] Excercise 103

Question 1

In the fig. 41, PO is a ray of light incident on a rectangular glass block.
(a)        Complete the path of the ray through the block.
(b)    In the diagram, mark the angle of incidence (i) and the angle of refraction (r) at the first interface. How is the refractive index of glass related to the angles I and r?
(c)    Mark angle of emergence by the letter e. How are the angles i and e related?
(d)    Which two rays are parallel to each other? Name them.
(e)        Indicate in the diagram the lateral displacement between the emergent ray and the incident ray.

Solution 1

Question 2
State two factors which affect the critical angle for a given pair of media.
Solution 2
Factors affecting the critical angle are:
(i) The colour (or wavelength) of light.
(ii) The temperature (on changing the temperature of medium, its refractive index changes).
Question 3
The critical angle for glass-air is 450 for the light of yellow colour. State whether it will be less than, equal to, or more than 450 for (i) red light, (ii) blue light?
Solution 3
The critical angle for a pair of media is:
(i) more than 45o for red light.
(ii) less than 45o for blue light.
Question 4

Fill in the blanks to complete the following sentences:
(a)    When light travels from a rarer to a denser medium, it bends ...........
(b)    When light travels from a denser to a rarer medium, it bends ............
(c)    The refractive index of glass with respect to air is 3/2. The refractive index of air with respect to glass will be...........

Solution 4

(a) towards the normal.
(b) away from the normal.
(c) 2/3.

Question 5
Define the terms critical angle and total internal reflection. State two conditions necessary for total internal reflection to occur. Write down the relation for the critical angle in terms of refractive index.
Solution 5
Critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90o.
Total internal reflection: When a ray of light travelling from an optically denser medium to an optically rare medium is incident at an angle greater than the critical angle for the pair of media in contact, the ray is totally reflected back into the denser medium. This phenomenon is referred as total internal reflection.
The two necessary conditions for total internal reflection to take place are:
1.    The light ray must proceed from denser to rarer medium.
2. Angle of incidence in denser medium should be greater than the critical angle for the pair of media in contact.
Relation between critical angle and r.i.: The critical angle can thus be calculated by taking the inverse-sine ratio of speed of light in denser medium and the speed of light in rarer.
Question 6
What do you mean by the statement 'the critical angle for diamond is 240'?
Solution 6
The critical angle for diamond is 24o; this indicates that diamond has high refractive index of 2.42. This low value of critical angle facilitates total internal reflection of each light ray entering the diamond at its multiple surfaces.
Question 7
Explain the meaning of the term refraction. A ray of monochromatic light is incident from air on a glass slab. Draw a ray diagram indicating the change in its path till it emerges out of the slab.
Solution 7
Refraction is the bending of light at the surface of separation, which takes when it passes from one optical medium to another optical medium with different optical densities.
Ray diagram showing refraction of light through a glass slab:
Question 8
State two uses of a total reflecting prism as a reflector in place of a plane mirror.
Solution 8
Two advantages of using total reflecting prism as a reflector in place of a plane mirror are:
(i) When total internal reflection occurs, the entire light (100%) is reflected back into the denser medium, whereas in ordinary reflection from plane mirror, some light is refracted or absorbed (i.e. reflection is partial).
(ii) Total reflecting prism gives a much brighter image than that obtained by using the plane mirror.
Question 9
Solution 9

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