ICSE Class 10 Physics Selina Solutions – Chapter 4: Refraction of Light at Plane Surfaces Exercise 4(A)

ICSE Class 10 Physics Selina Solutions – Chapter 4: Refraction of Light at Plane Surfaces Exercise 4(A)

If you are preparing for your ICSE Class 10 board exams, mastering Refraction of Light at Plane Surfaces is essential. Selina Solutions for Chapter 4 Exercise 4(A) provide clear, accurate, and step-by-step answers to help you understand the concepts better and score well.

Why Exercise 4(A) is Important?

Exercise 4(A) focuses on the laws of refraction, refractive index, and how light changes direction when passing from one medium to another. These are the basics you must understand before moving on to more complex numerical problems in later exercises. Many of these questions are directly or indirectly asked in exams, making this section vital for practice.

(A)  Multiple Choice Type

Question 1
In the diagram shown below, which one of the following statements is true?

(a) C is incident ray and A is reflected ray
(b) A is incident ray and D is refracted ray
(c) A is incident ray and B is refracted ray
(d) B is incident ray and C is refracted ray

Answer:

(b) A is incident ray and D is refracted ray

Explanation:
Light is going from air to water, bends towards the normal. So D is refracted.

Question 2
When a ray of light from air enters a denser medium, it:
(a) bends away from the normal
(b) bends towards the normal
(c) goes undeviated
(d) is reflected back

Answer:

(b) bends towards the normal

Explanation:
Light slows down in denser medium and bends towards the normal.

Question 3
A light ray does not bend at the boundary while passing from one medium to the other medium if the angle of incidence is:
(a)  0°                      (b)  45°
(c)  60°                    (d)  90°

Answer:

(a) 0°

Explanation:
A light ray incident normally (at 0°) to the surface does not bend even if the medium changes.

Question 4
Which of the following formulae is used to calculate the refractive index μ of the glass?

(a) \(\mu=\frac{sin\ A}{sin\ C}\)

(b) \(\mu=\frac{sin\ C}{sin\ A}\)

(c) \(\mu=\frac{sin\ B}{sin\ C}\)

(d) \(\mu=\frac{sin\ B}{sin\ A}\)

Answer:

(c) \(\mu=\frac{sin\ B}{sin\ C}\)

Explanation:

According to Snell’s law,  \(\mu=\frac{sin\ i}{sin\ r}\)

Question 5
The incorrect statement among the following is:
(a) When a ray of light gets refracted from a denser to a rarer medium, the speed of light decreases.
(b) The frequency of light does not change on refraction.
(c) The speed of light \(V\) in a medium is related to its wavelength \(λ\) and frequency  \(f\)  as .
(d) The incident ray, the refracted ray and the normal at the point of incidence all lie in one plane.

Answer:

(a) When a ray of light gets refracted from a denser to a rarer medium, the speed of light decreases.

Explanation:
In rarer medium, speed increases, not decreases.

Question 6
In refraction of light, the relation between incident angle \(i\) and refracted angle \(r\) is:
(a) \(i > r\)
(b) \(r > i\)
(c) \(i = r\)
(d) there is no fixed relation

Answer:

(d) there is no fixed relation

Explanation:

There is no fixed relation between the incident angle  and the refracted angle  because:

• When light travels from a rarer medium to a denser medium, it bends towards the normal, so angle r < angle i.

• When light travels from a denser medium to a rarer medium, it bends away from the normal, so angle r > angle i.

• When light is incident normally on the surface (i.e., at 0°), it passes undeviated, and angle r = angle i.

Question 7
The intensity of the refracted light is always ………… the intensity of incident light:
(a) greater than
(b) equal to
(c) less than
(d) none of the above

Answer:

(c) less than

Explanation: The intensity of the refracted light is always less than that of the incident light because a portion of the incident light is reflected at the boundary.

Question 8
The ray AO is shown in the figure. The path of refracted ray and emergent ray will be:

(1) the refracted ray will bend towards normal
(2) the refracted ray will bend away from normal
(3) the emergent ray will bend towards normal
(4) the emergent ray will bend away from normal

(a) (1) and (3)
(b) (1) and (4)
(c) (2) and (3)
(d) (2) and (4)

Answer:

(b) (1) and (4)

Explanation:
Since air is a rarer medium and glass is a denser medium, when a light ray passes from air into glass, it bends towards the normal. As the ray emerges from the glass back into air, it bends away from the normal.

Question 9

What is the deviation of a ray of light which is incident normally on the surface of two separating media:
(a) 0°                        (b) 45°
(c) 90°                      (d) 180°

Answer:

(a) 0°

Explanation: Normal incidence means no bending (angle = 0°).

Question 10
If refractive indices of medium 1 and 2 are same, the speed of light will:
(a) increase
(b) decrease
(c) remain the same
(d) none of the above

Answer:

(c) remain the same

Explanation:

If both media have the same refractive index, then light travels at the same speed in both. Since there’s no change in optical density, the light ray continues without any change in speed or direction.

Question 11
When light passes from a denser medium to a rarer medium, its wavelength:
(a) decreases
(b) increases
(c) remains the same
(d) none

Answer:

(b) increases

Explanation: Wavelength ∝ speed of light, which increases in rarer medium.

Question 12
Which of the following is correct when a ray of light passes from a rarer medium to a denser medium?
(1) speed will decrease
(2) frequency remains unchanged
(3) wavelength will decrease
(4) intensity will decrease

(a) (1)
(b) (1) and (4)
(c) (1), (2) and (4)
(d) all of the above

Answer:

(d) all of the above

Explanation:
When light passes from a rarer to a denser medium, its speed and wavelength decrease, frequency remains unchanged, and intensity decreases due to partial reflection, making option (d) correct.

Question 13
Refractive index of water is 4/3 and of glass is 3/2. Refractive index of glass with respect to water is:
(a) 9/8                         (b) 2
(c) 8/9                         (d) 1

Answer:

(a) 9/8

Explanation:

Given,

_aμ_w =  4/3

_aμ_g  = 3/2

_wμ_g \(=\frac{Refractive\ index\ of\ glass}{Refractive\ index\ of\ water}\)

Substituting we get,

_wμ_g \(=\ \frac{\frac{3}{2}}{\frac{4}{3}}=\frac{3\ \times\ 3}{2\ \times\ 4}=\ \frac{\mathbf{9}}{\mathbf{8}}\)

Question 14
The optical density of turpentine is higher than that of water, while its mass density is lower than water. The figure given below shows a layer of turpentine floating over water taken in a container. For which one of the four rays incident on turpentine in the path shown in the figure is correct?

(a) Ray 1
(b) Ray 2
(c) Ray 3
(d) Ray 4

Answer: 

(b) Ray 2

Explanation:
From a denser to rarer medium, light bends away from the normal.
So, Ray 2 correctly shows this behavior.

Question 15
With respect to light passing from air into water which of the following is correct?
(a) Angle of incidence is equal to the angle of refraction
(b) Angle of incidence is always greater than the angle of refraction
(c) Angle of incidence is always less than the angle of refraction
(d) Angle of incidence has no effect on the angle of refraction

Answer:

(b) Angle of incidence is always greater than the angle of refraction

Explanation:
Air → Water (rarer to denser), bends toward normal, so i > r

(B)  Very Short Questions

Question 1
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) Angle of refraction = 0°

(ii) Angle of deviation = 0°
(Since the light passes straight without bending when incident normally)

Question 2
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:

Yes, the speed of light changes in the other medium due to its different optical density, even though the ray does not bend.

Question 3
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:

• The quantities that differ are: Speed, Intensity and Wavelength.
• Frequency remains the same.

Question 4
A light ray passes from water to (i) air, and (ii) glass. In each case, state how does the speed of light change?

Answer:

(i) When light travels from water to air, its speed increases because it moves from a denser medium to a rarer one.

(ii) When light travels from water to glass, its speed decreases because it moves from a rarer medium to a denser one.

Question 5
(a) For which colour of white light, is the refractive index of a transparent medium
(i) the least
(ii) the most?
(b) Which colour of white light travels fastest in any medium except air?

Answer:

(a) (i)   Least = Red
(ii)  Most = Violet

(b) Red light travels fastest in any medium (except air).

Question 6
How does the refractive index of a medium depend on the wavelength of light used?

Answer:

Refractive index decreases with increase in wavelength.

Question 7
How does the refractive index of a medium depend on its temperature?

Answer:

As temperature increases, the refractive index decreases.

Question 8
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.

_aμ_w \(=\frac{Speed\ of\ light\ in\ air}{Speed\ of\ light\ in\ water}\)

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

_aμ_g \(=\frac{Speed\ of\ light\ in\ air}{Speed\ of\ light\ in\ glass}\)

So, the refractive index of glass with respect to water is given by \({{}_w\mu}_g=\frac{{{}_a\mu}_g}{{{}_a\mu}_w}\)

Question 9
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 ……………

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) Short Questions

Question 1
What do you understand by refraction of light?

Answer:

Refraction of light is the bending of a light ray when it passes from one transparent medium to another transparent medium. This bending occurs due to a change in the speed of light.

Question 2
What is the cause of refraction of light when it passes from one medium to another?

Answer:

The cause of refraction is the change in speed of light as it travels from one medium to another. When light travels from one medium to another with a different density, its speed changes, resulting in a change in its direction.

Question 3
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.

\(\frac{sin\ i}{sin\ r}=\ constant\ \mu\)

Question 4

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

Answer:

The refractive index of a medium is the ratio of the speed of light in vacuum (or air) to the speed of light in that medium:

\(\mu=\frac{Speed\ of\ light\ in\ vacuum\ or\ air\ (c)\ }{Speed\ of\ light\ in\ that\ medium\ (V)}\)

No, refractive index cannot be less than 1, because light always travels fastest in vacuum.

Question 5
(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) In vacuum, all colours of light travel at the same speed.
So, speed of violet light = speed of red light = 3 × 10⁸ m/s

(b) The refractive index (μ) is given by the formula: \(\mu=\frac{c}{v}\)
Where,
μ = refractive index of the medium
c = speed of light in vacuum or air
v = speed of light in the medium

Question 6
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:

(a) Speeds up:
(i) Example: Air
(ii) Refractive index is less than that of water.

(b) Slows down:
(i) Example: Glass
(ii) Refractive index is greater than that of water.

Question 7
What do you understand by the statement ‘the refractive index of glass is 1.5 for white light’?

Answer:

The refractive index of glass is 1.5. It means that light travels in air 1.5 times faster than in glass. 

OR, 

It means that the speed of light in glass is 1.5 times less than its speed in vacuum or air.

Question 8
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 relation 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 —

 \(\mu=\frac{Speed\ of\ light\ in\ vacuum\ or\ air\ \left(c\right)}{Speed\ of\ light\ in\ that\ medium\left(V\right)}\)

\(⇒\ 1.5\ =\ \frac{c}{V}\)

\(⇒\ c\ =\ 1.5\ V\)

(b) Let wavelength of light in glass be \(\lambda^\prime\)
so we get,

\(\mu\ =\ \frac{\lambda}{\lambda^{‘}}\)

\(⇒\ 1.5\ =\ \frac{\lambda}{\lambda^{‘}}\)

\(⇒\ \lambda^{‘}\ =\frac{\lambda}{1.5}\)

Question 9
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 refractive index will be different.
Blue light (shorter wavelength) bends more and has a higher refractive index than red light (longer wavelength). Hence, the refractive index of red and blue light in glass will be different.

Question 10
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 —

1. Wavelength of light:
   Refractive index decreases as wavelength increases.

2. Temperature:
Refractive index decreases with increase in temperature (due to decreased optical density).

Question 11
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 that of the glass at a certain temperature.

(ii) The refractive index of a medium (like glass or liquid) varies for light of different colors, so a single color of light is used.

Question 12
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:

The second image is formed due to reflections from both front and back surfaces of the glass. It appears brighter because the reflected rays are nearly parallel and close to the original image, thus undergoing less scattering or absorption than subsequent reflections.

(D) Long Questions

Question 1
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:

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

Question 2
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:

Question 3
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:

(b) 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.

 \(\frac {sin\ i}{sin\ r}\ =\ \) constant ₁μ₂

Question 4
The diagram alongside shows the refraction of a ray of light from air to a liquid.

(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)  Angle of incidence, ∠i = 90° – 30° = 60°

(ii) Angle of refraction, ∠r = 90° – 45° = 45°.

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

\({{}_{air}\mu}_{liquid}=\frac{\sin\;i}{\sin\;r}=\frac{\sin\;60^\circ\;}{\sin\;45^\circ}=\frac{\displaystyle\frac{\sqrt3}2}{\displaystyle\frac1{\sqrt2}}=\frac{\sqrt3}{\sqrt2}=1.22\)

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

Question 5
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:

Lateral displacement is the perpendicular distance between the incident ray and the emergent ray.

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

Question 6
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:

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

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

Question 7
In the adjacent diagram, AO is a ray of light incident on a rectangular glass slab.

(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?
(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. 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.

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.

\(\frac{\sin\;i}{\sin\;r}=\) constant ₁μ₂

(c) The angle of emergence (e) is marked in diagram.

Relation between angles i and e:

angle of incidence (i) = angle of emergence (e) 

(d) The incident ray AO is parallel to the  emergent ray BC.

(e) The lateral displacement between the incident ray and the emergent ray is denoted by CD.

One factor that affect lateral displacement is the thickness of the medium. Greater thickness of the medium results in increased lateral displacement.

Question 8
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.

(c) Redraw the diagram if plane mirror becomes normal to the refracted ray inside the liquid. State the principle use.

Answer:

(a) Refractive index of the liquid is given by

\({{}_{air}\mu}_{liquid}=\frac{\sin\;i}{\sin\;r}=\frac{\sin\;45^\circ\;}{\sin\;30^\circ}=\frac{\displaystyle\frac1{\sqrt2}}{\displaystyle\frac{1}{2}}=\sqrt2=1.414\)

(b)

(c)

The principle of reversibility is applied here.

(E) Numericals

Question 1
The speed of light in air is \(3\times{10}^8\ m\ s^{-1}\). Calculate the speed of light in glass. The refractive index of glass is 1.5.

Answer:

Given,

Speed of light in air, C \(=\ 3\times{10}^8\ m\ s^{-1}\)

Refractive index of glass, \mu=1.5

We know that,

\(\mu=\frac{Speed\ of\ light\ in\ vacuum\ or\ air\ (C)}{Speed\ of\ light\ in\ glass\ (V)}\)

\(\Rightarrow\ 1.5=\frac{3\ \times\ {10}^8}{V}\)

\(\Rightarrow\ V=\frac{3\ \times\ {10}^8}{1.5}\)

\(\Rightarrow\ V=2\times{10}^8\ m\ s^{-1}\)

Hence, Speed of light in glass \(=2\times{10}^8\ m\ s^{-1}\).

Question 2
The speed of light in diamond is \(125,000\ km\ s^{-1}\). What is the refractive index? (Speed of light in air \(=\ 3\times{10}^8\ m\ s^{-1}\)).

Answer:

Given,

Speed of light in air, C \(=\ 3\times{10}^8\ m\ s^{-1}\)

Speed of light in diamond, V \(=\ 125,000\ km\ s^{-1}=\ 125\times{10}^6\ m\ s^{-1}\)

We know that,

\(\mu=\frac{Speed\ of\ light\ in\ vacuum\ or\ air\ (C)}{Speed\ of\ light\ in\ diamond\ (V)}\)

\(\Rightarrow\ \mu=\frac{3\times{10}^8}{125\times{10}^6}=\frac{300}{125}=2.4\)

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:

Refractive index of water w.r.t air, \({{}_a\mu}_w=\frac43\)

Refractive index of air w.r.t water,

\({{}_w\mu}_a=\frac1{{{}_a\mu}_w}\)

\({{}_w\mu}_a=\frac1{\displaystyle\frac43}=\frac34=0.75\)

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

Question 4
A ray of light of wavelength 6600 Å suffers refraction from air to glass. Taking _aμ_g = 3/2, find the wavelength of light in glass.

Answer:

Given,

Refractive index of glass with respect to air is given by _aμ_g \(=\frac{3}{2}\)

Wavelength of light in air = 6600 Å

We know that,

\(\frac1{{{}_a\mu}_g}\ =\ \frac{wavelength\ of\ light\ in\ air\ }{wavelength\ of\ light\ in\ glass}\)

\(\Rightarrow\ \frac{3}{2}=\frac{6600\ }{wavelength\ of\ light\ in\ glass}\)

\(\Rightarrow\) Wavelength of light in glass \(\ =\ 6600\ \times\ \frac{2}{3}\)

\(\Rightarrow\) Wavelength of light in glass \(=\ 4400\)

Hence, wavelength of light in glass  \(=\  4400\ Å\)

Download Free Selina Concise Physics Class 10 Refraction of Light at Plane Surfaces Solutions

You can download free PDF solutions for Selina Class 10 Physics Exercise 4(A) to revise offline. These solutions are exam-ready and designed by subject experts.

Tips for Scoring Better in Refraction Questions

• Memorize the laws of refraction word-for-word for theory questions.

• Draw clear and labeled ray diagrams.

• Revise formulas for refractive index and understand how to apply them in different situations.

Benefits of Using Selina Solutions for Exercise 4(A)

• Simplified Explanations – Each solution is broken down so you can grasp the logic behind every step.

• Board Exam Alignment – Solutions follow the ICSE Class 10 syllabus and marking scheme.

• Strong Concept Building – Understanding Exercise 4(A) helps in solving numerical questions in Exercises 4(B), 4(C), and 4(D) with ease.