ICSE Class 10 Chapter 4 Physics "Refraction of Light at Plane Surface:" Consise physics Selina Solution 4(A) & 4(B) -

ICSE CLASS 10 Chapter 4 selina solution concise physics

Question 1

What do you understand by refraction of light?

Answer

The change in the direction of the path of light, when it passes from one transparent medium to another transparent medium, is called refraction of light. The refraction of light is essentially a surface phenomenon.

Question 2

Draw diagrams to show the refraction of light from (i) air to glass, (ii) glass to air. In each diagram, label the incident ray, refracted ray, the angle of incidence (i) and the angle of refraction (r).

Answer

(i) The below ray diagram shows the refraction of light from air to glass:

Draw diagram to show the refraction of light from air to glass. Label incident ray, refracted ray, angle of incidence, angle of refraction. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(ii) The below ray diagram shows the refraction of light from glass to air:

Question 3

A ray of light is incident normally on a plane glass slab. What will be (i) the angle of refraction and (ii) the angle of deviation for the ray?

Answer

(i) The ray of light incident normally on a plane glass slab passes undeviated, ( i.e. such a ray suffers no bending at the surface). Thus, if angle of incidence ∠i = 0°, then the angle of refraction ∠r = 0°.

(ii) In the case when ray of light is incident normally on a plane glass slab, it passes undeviated. Hence, the angle of deviation is 0°.

Question 4

An obliquely incident light ray bends at the surface due to change in speed, when passing from one medium to other. The ray does not bend when it is incident normally. Will the ray have different speed in the other medium?

Answer

We observe that, when a ray of light passing from one medium to other is obliquely incident it bends at the surface due to change in speed in going from one medium to another.

However, for normal incidence from one medium to another, angle of incidence ∠i = 0° then the angle of refraction ∠r = 0°. Hence the speed of light changes but the direction of light does not change.

Question 5

What is the cause of refraction of light when it passes from one medium to another?

Answer

When a ray of light passes from one medium to another medium, its direction (or path) changes because of the change in speed of light in going from one medium to another.

In passing from one medium to other, if light slows down, it bends towards the normal and if light speeds up, it bends away from the normal.

If the speed of light remains same in passing from one medium to another, the ray of light does not bend then it passes undeviated.

Hence, we can say that the cause of refraction of light is that its speed is different in different mediums.

Question 6

A light ray suffers reflection and refraction at the boundary in passing from air to water. Draw a neat labelled diagram to show it.

Answer

Air is a rarer medium while water is denser medium than air. Therefore, when a ray of light travels from air to water (i.e rarer to denser) it bends towards the normal. Below diagram shows reflection and refraction suffered by a light ray at the boundary in passing from air to water:

A light ray suffers reflection and refraction at the boundary in passing from air to water. Draw a neat labelled diagram to show it. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

Question 7

A ray of light passes from medium 1 to medium 2. Which of the following quantities of the refracted ray will differ from that of the incident ray: speed, intensity, frequency and wavelength?

Answer

Speed, intensity and wavelength are the quantities which will differ for a refracted ray in comparison to those of a incident ray.

The frequency of light depends on the source of light, so it does not change on refraction.

When a ray of light gets refracted from a rarer to a denser medium, the speed of light decreases; while if it is refracted from a denser to a rarer medium, the speed of light increases.

When light passes from a rarer to a denser medium, the wavelength decreases and when it passes from denser to rarer medium, its wavelength increases.

Question 8

State the Snell’s laws of refraction of light.

Answer

The Snell’s laws of refraction of light are:

(i) The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.

(ii) The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is constant for the pair of given media.

$sin r sin i = constant_{1} μ_{2}$

The constant ₁μ₂ is called the refractive index of the second medium with respect to the first medium.

Question 9

Define the term refractive index of a medium. Can it be less than 1?

Answer

The refractive index of the second medium with respect to the first medium is defined as the ratio of the sine of the angle of incidence in the first medium to the sine of the angle of refraction in the second medium.

$μ = Speed of light in that medium (V) Speed of light in vacuum or air(c)$

The refractive index of a transparent medium is always greater than 1 (it cannot be less than 1) because the speed of light in any medium is always less than that in vacuum ( i.e. V < c ) and μ = 1 for air or vacuum.

Question 10

(a) Compare the speeds of light of wavelength 4000Å (i.e. violet light) and 8000Å (i.e. red light) in vacuum.

(b) How is the refractive index of a medium related to the speed of light in it and in vacuum or air?

Answer

(a) As we know that light is an electromagnetic wave and it travels with same speed for all wavelength, which is equal to 3 x 10⁸.

Hence, the ratio of speeds of light of wavelength 4000Å (i.e. violet light) and 8000Å (i.e. red light) in vacuum = 1:1.

(b) The refractive index of a medium is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.

$μ = Speed of light in that medium (V) Speed of light in vacuum or air (c)$

Denser medium has a higher refractive index. Hence, the speed of light in such medium is lower in comparison to the speed of light in a medium which has a lower refractive index.

Question 11

A light ray passes from water to (i) air, and (ii) glass. In each case, state how does the speed of light change?

Answer

As we know,

Refractive index of air, μ_a = 1.0003

Refractive index of water, μ_w = 1.33

Refractive index of ordinary glass, μ_g = 1.5

With the help of the above values, we can see that μ_g > μ_w > μ_a

When a ray of light gets refracted from a rarer to a denser medium, the speed of light decreases, while if it is refracted from a denser to a rarer medium, the speed of light increases.

So on the basis of above information, we can say that —

(i) When a ray of light passes from water to air, its speed increases as it is moving from a denser to a rarer medium.

(ii) When a ray of light passes from water to glass, its speed decreases as it is moving from a rarer to a denser medium.

Question 12

A light ray in passing from water to a medium (a) speeds up (b) slows down. In each case, (i) give one example of the medium, (ii) state whether the refractive index of medium is equal to, less than or greater than the refractive index of water.

Answer

As we know that, when a ray of light gets refracted from a rarer to a denser medium, the speed of light decreases, while if it is refracted from a denser to a rarer medium, the speed of light increases.

(a) (i) When a ray of light passes from water to air, its speed increases as it is moving from a denser to a rarer medium.

(b) (i) When a ray of light passes from water to glass, its speed decreases as it is moving from a rarer to a denser medium.

(a) (ii) The refractive index of the medium (air) is less than that of water as the speed of refracted ray increases.

(b) (ii) The refractive index of the medium (glass) is more than that of water as the speed of refracted ray decreases.

Question 13

What do you understand by the statement ‘the refractive index of glass is 1.5 for white light’?

Answer

This statement indicates that white light travels in air 1.5 times faster than in glass or speed of light in glass is $1.51$ times the speed of light in air.

Question 14

A monochromatic ray of light passes from air to glass. The wavelength of light in air is λ, the speed of light in air is c and in glass is V. If the refractive index of glass is 1.5, write down,

(a) the relationship between c and V,

(b) the wavelength of light in the glass.

Answer

(a) The relation between the speed of light in air c and in glass V is given by —

$μ = Speed of light in that medium (V) Speed of light in vacuum or air (c) 1.5 = V c \Rightarrow c = 1.5 V$

(b) Let wavelength of light in glass be λ₁ so we get,

$μ = λ ^{1} λ 1.5 = λ ^{1} λ λ_{1} = 1.5 λ$

Question 15

A boy uses blue colour of light to find the refractive index of glass. He then repeats the experiment using red colour of light. Will the refractive index be the same or different in the two cases? Give a reason to support your answer.

Answer

The speed of blue light in glass is less that of red light.

i.e. c_b < c_r

We know that,

$μ = Speed of light in glass (V) Speed of light in vacuum or air (c)$

Hence, the refractive index of blue light is greater than that of red light.

i.e. μ_b > μ_r.

Hence, the refractive index of red and blue light in glass will be different.

Question 16a

For which colour of white light, is the refractive index of a transparent medium

(i) the least

(ii) the most?

Answer

(i) The speed of red light is maximum. Therefore, the refractive index of a transparent medium is least for red colour.

(ii) The speed of violet light is least. Therefore, the refractive index of a transparent medium is most for violet colour.

Question 16b

Which colour of light travels fastest in any medium except air?

Answer

The wavelength of visible light increases from violet to red colour. It is highest for red colour. Hence, red colour travels fastest in any medium except air.

Question 17

Name two factors on which the refractive index of a medium depends? State how does it depends on the factors stated by you.

Answer

The factors on which the refractive index of a medium depends are as follows —

(i) Nature of the medium (on the basis of speed of light) — Less the speed of light in the medium as compared to that in air, more is the refractive index of the medium.

We know that,

$μ = Speed of light in glass (V) Speed of light in vacuum or air (c)$

V_glass = 2 x 10 ⁸ ms^-1, μ_glass = 1.5

V_water = 2.25 x 10 ⁸ ms^-1, μ_water = 1.33

(ii) Physical condition such as temperature — With an increase in temperature, the speed of light in medium increases, so the refractive index of the medium decreases.

Question 18

How does the refractive index of a medium depend on the wavelength of light used?

Answer

In a given medium, the speed of red light is maximum. Therefore, the refractive index of that medium is maximum for violet light and least for red light. (i.e. μ_violet > μ_red).

The wavelength of visible light increases from violet to red end, so the refractive index of a medium decreases with increase in wavelength of light used.

Hence, the refractive index of a medium for violet light which has the least wavelength is greater than that for red light which has the greatest wavelength.

Question 19

How does the refractive index of a medium depend on its temperature?

Answer

With increase in temperature, the speed of light in a medium increases, so the refractive index of the medium decreases.

Question 20

Light of a single colour is passed through a liquid having a piece of glass suspended in it. On changing the temperature of the liquid, at a particular temperature, the glass piece is not seen.

(i) When is the glass piece not seen?

(ii) Why is the light of a single colour used?

Answer

(i) The glass piece is not seen when the refractive index of the liquid becomes equal to the refractive index of glass.

(ii) The light of a single colour is used as the refractive index of a medium (glass or liquid) is different for the light of different colours.

Question 21

In the figure below, a ray of light A incident from air suffers partial reflection and refraction at the boundary of water.

(a) Complete the diagram showing (i) the reflected ray B and (ii) the refracted ray C.

(b) How are the angles of incidence i and refraction r related?

Answer

(a) Below is the completed diagram with the refracted ray B and the reflected ray C labelled:

Completed diagram of a ray of light A incident from air suffering partial reflection and refraction at the boundary of water with reflected ray B and refracted ray C labelled. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(b) The relation between angle of incidence i and the angle of refraction r is given by Snell’s law as,

The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is constant for the pair of given media.

$sin r Sin i = constant_{1} μ_{2}$

The constant ₁μ₂ is called the refractive index of the second medium with respect to the first medium

$sin r sin i = μ w$

Question 22

The diagram alongside shows the refraction of a ray of light from air to a liquid.

The diagram shows the refraction of a ray of light from air to a liquid. Write the values of angle of incidence, angle of refraction. Use Snell's law to find refractive index of liquid with respect to air. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(a) Write the values of (i) angle of incidence, and (ii) angle of refraction.

(b) Use Snell’s law to find the refractive index of liquid with respect to air.

Answer

(a) (i) The angle of incidence is the angle which the incident ray makes with the normal. Hence, ∠i = 90° – 30° = 60°.

(ii) Angle of refraction is the angle which the refracted ray makes with the normal Hence, ∠r = 90° – 45° = 45°.

(b) We know that according to Snell’s law,

$_{air} μ_{liquid} = sin r sin i = sin 45 ° sin 60 ° = 2 1 2 3 = 2 3 = 1.22$

Hence, the refractive index of liquid with respect to air is 1.22

Question 23

The refractive index of water with respect to air is _aμ_w and of glass with respect to air is _aμ_g. Express the refractive index of glass with respect to water.

Answer

The refractive index of water with respect to air is _aμ_w.

$_{a} μ_{w} = Speed of light in water Speed of light in air$

The refractive index of glass with respect to air is _aμ_g.

$_{a} μ_{g} = Speed of light in glass Speed of light in air$

So, the refractive index of glass with respect to water is given by

$_{w} μ_{g} =^{a} μ ^{w} ^{a} μ ^{g}$

Question 24

What is lateral displacement? Draw a ray diagram showing the lateral displacement of a ray of light when it passes through a parallel-sided glass slab.

Answer

The perpendicular distance between the incident ray and the emergent ray is defined as lateral displacement. This shift depends upon the angle of incidence, the angle of refraction and the thickness of the medium.

Below diagram shows the lateral displacement of a ray of light when it passes through a parallel-sided glass slab:

Draw a ray diagram showing the lateral displacement of a ray of light when it passes through a parallel-sided glass slab. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

Question 25

A ray of light strikes the surface at a rectangular glass slab such that the angle of incidence in air is (i) 0°, (ii) 45°. In each case, draw a diagram to show the path taken by the ray as it passes through the glass slab and emerges from it.

Answer

(i) Below is the labelled diagram for ray of light at angle of incidence 0°:

A ray of light strikes the surface at a rectangular glass slab such that the angle of incidence in air is (i) 0°, (ii) 45°. In each case, draw a diagram to show the path taken by the ray as it passes through the glass slab and emerges from it. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(ii) Below is the labelled diagram for ray of light at angle of incidence 45°:

Question 26

In the adjacent diagram, AO is a ray of light incident on a rectangular glass slab.

In the diagram, AO is a ray of light incident on a rectangular glass slab. Complete the path of the ray until it emerges out of the slab. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(a) Complete the path of the ray until it emerges out of the slab.

(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 angles i and r?

(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. State one factor that affects the lateral displacement.

Answer

(a) The below diagram shows the path of the ray until it emerges out of the slab:

(b) In diagram, the angle of incidence (i) and the angle of refraction (r) are marked.

Refractive index of glass μ is related to the angle of incidence (i) and angle of reflection (r) by Snell’s second law and is represented as —

The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is constant for the pair of given media

$sin r sin i = constant_{1} μ_{2}$

The constant ₁μ₂ is called the refractive index of the second medium with respect to the first medium.

As the refraction occurs at two parallel surfaces PQ and RS, therefore angle MOB = angle N₁BO and by the principle of reversibility of the path of a light ray angle of emergence (e) is equal to the angle of incidence (i).

(d) As the refraction occurs at two parallel surfaces PQ and RS, therefore angle MOB = angle N₁BO and by the principle of reversibility of the path of a light ray angle of emergence (e) is equal to the angle of incidence (i).

Hence, the emergent ray BC is parallel to the incident ray AO.

(e) The lateral displacement between the incident ray and the emergent ray is represented by XY.

One of the factors that affects lateral displacement is the the thickness of the medium. More the thickness of the medium more is the lateral displacement.

Question 27

A ray of green light enters a liquid from air, as shown in the figure. The angle 1 is 45° and angle 2 is 30°.

(a) Find the refractive index of liquid.

(b) Show in the diagram the path of the ray after it strikes the mirror and re-enters in air. Mark in the diagram the angles wherever necessary.

Answer

(a) Refractive index of the liquid is given by Snell’s law and is shown as below,

_airμ_liquid = $sin r sin i$

_airμ_liquid = $sin 30 ° sin 45 °$

$= 21 2 1 = 2 2 = 2 = 1.414$

(b) Below labelled diagram shows the path of the ray after it strikes the mirror and re-enters in air:

The principle of reversibility is used.

Question 28

When an illuminated object is held in front of a thick plane glass mirror, several images are seen, out of which the second image is the brightest. Give reason.

Answer

As shown in the figure, if an illuminated object is held in front of a thick plane glass mirror, and is viewed obliquely, a number of images are seen and out of these the second image is the brightest.

The reason for this is that when a ray of the light falls on the surface of the mirror from the source, a small part of light (nearly 4%) is reflected forming a faint virtual image, while a large part of light (nearly 96%) is refracted inside the glass.

Now, we can see that this ray is strongly reflected back by the silvered surface inside the glass. This ray is then partially refracted in air and this refracted ray forms another virtual image.

This image is the brightest image because it is due to the light suffering a strong first reflection at the silvered surface.

Question 29

Fill in the blanks to complete the following sentences —

(a) When light travels from a rarer to a denser medium, its speed ………………….

(b) When light travels from a denser to a rarer medium, its speed ………………….

(c) The refractive index of glass with respect to air is 3/2. The refractive index of air with respect to glass will be ………………….

Answer

(a) When light travels from a rarer to a denser medium, its speed decreases.

(b) When light travels from a denser to a rarer medium, its speed increases.

(c) The refractive index of glass with respect to air is 3/2. The refractive index of air with respect to glass will be 2/3.

Multiple Choice Type

Question 1

When a ray of light from air enters a denser medium, it:

bends away from the normal
bends towards the normal ✓
goes undeviated
is reflected back

Question 2

A light ray does not bend at the boundary in passing from one medium to the other medium if the angle of incidence is:

0° ✓
45°
60°
90°

Question 3

The highest refractive index is of:

Glass
Water
Diamond ✓
Ruby

Numericals

Question 1

The speed of light in air is 3 x 10⁸ m s^-1. Calculate the speed of light in glass. The refractive index of glass is 1.5.

Answer

As we know,

$μ = Speed of light in glass (V) Speed of light in vacuum or air (c)$

Given,

Refractive index of glass, μ = 1.5

Speed of light in air, C = 3 × 10⁸ m / s

Substituting the values in the formula above we get,

$1.5 = V 3 \times 10 ^{8} V = 1.53 \times 10 ^{8} \Rightarrow V = 2 \times 1 0^{8}$

Hence, Speed of light in glass = 2 × 10⁸ m s^-1

Question 2

The speed of light in diamond is 125,000 km s^-1. What is the refractive index? (Speed of light in air = 3 x 10⁸ m s^-1).

Answer

As we know,

$μ = Speed of light in diamond (V) Speed of light in vacuum or air (c)$

Given,

Speed of light in diamond = 125,000 km s^-1 = 125 x 10⁶ m/s

Speed of light in air, c = 3 × 10⁸ ms^-1

Substituting the values in the formula above we get,

$μ = 125 \times 10 ^{6} 3 \times 10 ^{8} μ = 1253 \times 10 ^{2} μ = 125300 \Rightarrow μ = 2.4$

Hence, refractive index = 2.4

Question 3

The refractive index of water with respect to air is 4/3. What is the refractive index of air with respect to water?

Answer

As it is given that,

_aμ_w = $34$

And we know that —

_wμ_a = $34 1$

_wμ_a = $43$

_wμ_a = 0.75

Hence, the refractive index of air with respect to water is 0.75

Question 4

A ray of light of wavelength 5400 Å suffers refraction from air to glass. Taking _aμ_g = 3/2, find the wavelength of light in glass.

Answer

As we know that,

$_{a} μ_{g} = wavelength of light in glass wavelength of light in air$

Given,

Refractive index of glass with respect to air is given by _aμ_g = $23$

Wavelength of light in air = 5400 Å

Substituting the values in the formula above we get,

$23 = wavelength of light in glass wavelength of light in air 23 = wavelength of light in glass 5400 wavelength of light in glass = 23 5400 wavelength of light in glass = 5400 \times 32 wavelength of light in glass = 3600$

Hence, wavelength of light in glass = 3600 Å

Exercise 4(B)

Question 1

What is a prism? With the help of a diagram of the principal section of a prism, indicate its refracting surfaces, refracting angle and base.

Answer

A prism is defined as a transparent medium bounded by five plane surfaces with a triangular cross section. The below diagram shows the principal section of a prism with its refracting surfaces, refracting angle and base labelled:

What is a prism? With the help of a diagram of the principal section of a prism, indicate its refracting surfaces, refracting angle and base. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

Question 2

The diagrams (a) and (b) in figure below show the refraction of a ray of light of single colour through a prism and a parallel sided glass slab, respectively.

(a)

The diagrams (a) and (b) show the refraction of a ray of light of single colour through a prism and a parallel sided glass slab, respectively. In each diagram, label the incident, refracted, emergent rays and the angle of deviation. In what way the direction of emergent ray in the two cases differ with respect to the incident ray? Explain your answer. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(b)

(i) In each diagram, label the incident, refracted, emergent rays and the angle of deviation.

(ii) In what way the direction of the emergent ray in the two cases differ with respect to the incident ray? Explain your answer.

Answer

(i) The below diagram shows the refraction of a ray of light of single colour through a prism with the incident, refracted, emergent rays and the angle of deviation properly labelled:

The below diagram shows the refraction of a ray of light of single colour through a parallel sided glass slab with the incident, refracted, emergent rays and the angle of deviation properly labelled:

(ii) In a prism, the refraction of light occurs at two inclined faces, so the emergent ray is not parallel to the incident ray but it is deviated towards the base of the prism. On the other hand, in a parallel sided glass slab, the refraction of light occurs at two parallel faces, so the emergent ray is parallel to the incident ray with a lateral displacement.

Question 3

Define the term angle of deviation.

Answer

Angle of deviation is the angle between the direction of incident ray and the emergent ray. It is represented by the greek alphabet δ (delta).

Question 4

Complete the following sentence:

Angle of deviation is the angle which the …… ray makes with the direction of …… ray.

Answer

Angle of deviation is the angle which the emergent ray makes with the direction of incident ray.

Question 5

What do you understand by the deviation produced by a prism? Why is it caused? State three factors on which the angle of deviation depends.

Answer

In a prism, the ray of light suffers refraction at two inclined faces. In each refraction, the ray bends towards the base of the prism. The below diagram shows the refraction of a ray of light of single colour through a prism:

What do you understand by the deviation produced by a prism? Why is it caused? State three factors on which the angle of deviation depends. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

At the first face AB, the ray suffers a deviation equal to δ₁. Then, at the second face AC, the ray PQ suffers a deviation δ₂. Thus, the prism has produced a deviation which is the angle between the direction of incident ray and the emergent ray. This is called the angle of deviation produced by a prism.

The angle of deviation is caused as the ray passing through a prism suffers refraction at two inclined planes.

The three factors on which the angle of deviation depends are as follows:

The angle of incidence (i)
The material of prism (i.e on refractive index μ)
The angle of prism (A)

Question 6

(a) How does the angle of deviation produced by a prism change with increase in the angle of incidence. Draw a curve showing the variation in the angle of deviation with the angle of incidence at a prism surface.

(b) Using the curve in part (a) above, how would you infer that for a given prism, the angle of minimum deviation ?_min is unique for light of a given wavelength.

Answer

(a) It is experimentally observed that as the angle of incidence increases, the angle of deviation first decreases, reaches to a minimum value for a certain angle of incidence and then on further increasing the angle of incidence, the angle of deviation begins to increase. The below curve shows the variation in the angle of deviation with the angle of incidence at a prism surface:

How does the angle of deviation produced by a prism change with increase in the angle of incidence. Draw a curve showing the variation in the angle of deviation with the angle of incidence at a prism surface. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

(b) For a given prism and given colour of light, ?_min (angle of minimum deviation) is unique since only one horizontal line can be drawn parallel to i – axis at the lowest point of i – ? curve i.e only for one value of angle of incidence i, the refracted ray inside the prism is parallel to its base.

Question 7

State whether the following statement is ‘true’ or ‘false’.

The deviation produced by a prism is independent of the angle of incidence and is same for all the colours of light.

Answer

The statement given above is False.

Question 8

How does the deviation produced by a prism depend on

(i) the refractive index of its material, and

(ii) the wavelength of incident light

Answer

(i) For a given angle of incidence, the prism with a higher refractive index produces greater deviation than the prism which has a lower refractive index.

For example — A flint glass prism produces more deviation than a crown glass prism for same refracting angle since μ_flint > μ_crown

(ii) The refractive index of a given transparent medium is different for the light of different colours. It decreases with the increase in the wavelength of light.

Thus, the refractive index of the material of a prism for visible light is maximum for the violet colour and minimum for the red colour.

Consequently, a given prism deviates violet light the most and red light the least.

(i.e. δ _violet > δ _red since μ_violet > μ_red )

Question 9

How does the angle of minimum deviation produced by a prism change with increase in (i) the wavelength of incident light and (ii) the refracting angle of the prism?

Answer

(i) The angle of minimum deviation produced by a prism decreases with increase in the wavelength of incident light.

(ii) The angle of minimum deviation produced by a prism increases with increase in the refracting angle of the prism.

Question 10

Write a relation for the angle of deviation (?) for a ray of light passing through an equilateral prism in terms of angle of incidence (i₁), angle of emergence (i₂) and angle of prism (A).

Answer

The relation between the angle of incident (i₁), angle of emergence (i₂), angle of prism (A) and angle of deviation (?) for a ray of light passing through an equilateral prism is given by —

? = i₁ + i₂ – A

Question 11

A ray of light incident at an angle of incidence i₁ passes through an equilateral glass prism such that the refracted ray inside the prism is parallel to its base and emerges at an angle of emergence i₂.

(i) How is the angle of emergence ‘i₂‘ related to the angle of incidence ‘i₁‘.

(ii) What can you say about the angle of deviation in such a situation?

Answer

(i) When a ray of light incident at an angle of incidence i₁ passes through an equilateral glass prism such that the refracted ray inside the prism is parallel to its base and emerges at an angle of emergence i₂ then angle of incidence i₁ is equal to the angle of emergence i₂.

Hence,

i₁ = i₂

(ii) When a ray of light incident at an angle of incidence i₁ passes through an equilateral glass prism such that the refracted ray inside the prism is parallel to its base and emerges at an angle of emergence i₂ then angle of deviation is minimum.

Question 12

Draw a ray diagram to show the refraction of a monochromatic ray through a prism when it suffers minimum deviation. How is the angle of emergence related to the angle of incidence in this position.

Answer

Below diagram shows the refraction of a monochromatic light ray through a prism when it suffers minimum deviation:

Draw a ray diagram to show the refraction of a monochromatic ray through a prism when it suffers minimum deviation. How is the angle of emergence related to the angle of incidence in this position. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

In an equilateral prism, when the prism is in minimum deviation, the angle of incidence i₁ is equal to the angle of emergence i₂.

Hence,

Angle of emergence (i₂) = Angle of incidence (i₁).

Question 13

A light ray of yellow colour is incident on an equilateral glass prism at an angle of incidence equal to 48° and suffers minimum deviation by an angle of 36°.

(i) What will be the angle of emergence?

(ii) If the angle of incidence is changed to (a) 30°, (b) 60°, state in each case whether the angle of deviation will be equal to less than or more than 36°?

Answer

(i) As we know that in an equilateral glass prism the ray suffers minimum deviation.

So, angle of incidence i₁ is equal to angle of emergence i₂

Given, angle of incidence i₁ = 48°

Applying the rule given above, we get,

i₁ = i₂
⇒ i₂ = 48°

Hence, angle of emergence i₂ = 48°

(ii) (a) As the minimum angle of deviation is 36° for yellow light, at an incident angle of 48° so for any angle of incidence other than 48° the angle of deviation will be more than 36°.

Hence, when the angle of incidence is changed to 30°, the angle of deviation will be more than 36°.

(b) As the minimum angle of deviation is 36° for yellow light, at an incident angle of 48° so for any angle of incidence other than 48° the angle of deviation will be more than 36°.

Hence, when the angle of incidence is changed to 60°, the angle of deviation will be more than 36°.

Question 14

Name the colour of white light which is deviated

(i) the most,

(ii) the least, on passing through a prism.

Answer

(i) On passing through a prism, violet colour will deviate the most.

(ii) On passing through a prism, red colour will deviate the least

Question 15

Which of the two prisms, A made of crown glass and B made of flint glass, deviates a ray of light more?

Answer

For a given angle of incidence, the prism with a higher refractive index produces greater deviation than the prism which has a lower refractive index.

As refractive index of flint glass prism (B) is more than a crown glass prism (A) (i.e. μ_flint > μ_crown)

Hence, a flint glass prism (B) produces more deviation than a crown glass prism (A).

Question 16

How does the angle of deviation depend on refracting angle of the prism?

Answer

The angle of deviation (?) increases with increase in the angle of prism (A).

Question 17

An object is viewed through a glass prism with its vertex pointing upwards. Draw a ray diagram to show the formation of its image as seen by the observer.

Answer

Below ray diagram shows the formation of the image of an object when seen by an observer through a glass prism with its vertex pointing upwards:

An object is viewed through a glass prism with its vertex pointing upwards. Draw a ray diagram to show the formation of its image as seen by the observer. Refraction of light at plane surfaces, Concise Physics Class 10 Solutions.

Question 18

A ray of light is normally incident on one face of an equilateral glass prism.

Answer the following —

(a) What is the angle of incidence on the first face of the prism?

(b) What is the angle of refraction from the first face of the prism?

(d) Will the light ray suffer minimum deviation by the prism?

Answer

(a) The angle of incidence on the first face of the prism when a ray of light is normally incident is 0°.

(b) The angle of refraction from the first face of the prism r₁ = 0° as the angle of refraction of a normally incident ray is always 0°.

Given, r₁ = 0°.

Hence, taking into consideration the two conditions we get,

the angle of incidence at the second face of the prism, equal to 60°.

(d) No, the light ray will not suffer minimum deviation as the angle of incidence is 0°.

Multiple Choice Type

Question 1

In refraction of light through a prism, the light ray:

suffers refraction only at one face of the prism
emerges out from the prism in a direction parallel to the incident ray
bends at both the surfaces of the prism towards its base ✓
bends at both the surfaces of prism opposite to its base.

Question 2

A ray of light suffers refraction through an equilateral prism. The deviation produced by the prism does not depend on the:

angle of incidence
colour of light
material of prism
size of prism ✓

ICSE CLASS 10 Chapter 4 selina solution concise physics

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