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

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

If you want to strengthen your understanding of refraction concepts, Selina Solutions for ICSE Class 10 Physics Chapter 4 Exercise 4(B) is a great place to start. This section focuses mainly on numerical problems related to refractive index, speed of light in different media, and the application of formulas you’ve learned in earlier exercises.

Why Exercise 4(B) Matters

Exercise 4(B) builds on the basics covered in Exercise 4(A). Here, you’ll find Refraction of Light at Plane Surfaces problems that test both your conceptual clarity and calculation skills. These questions are exam-relevant and help you learn how to apply the laws of refraction to real-life scenarios and numerical problems.

Exercise 4(B): (A) Multiple Choice Type

(Choose the correct answer from the options given below)

Question 1
In refraction of light through a prism, the light ray:
(a) suffers refraction only at one face of the prism
(b) emerges out from the prism in a direction parallel to the incident ray
(c) bends at both the surfaces of the prism towards its base
(d) bends at both the surfaces of prism opposite to its base.

Answer:

(c) bends at both the surfaces of the prism towards its base

Explanation:

• When light passes through a prism, it bends twice — once at the first surface (air to prism) and again at the second surface (prism to air).

• It always bends towards the base of the prism at both surfaces.

Question 2
A ray of light suffers refraction through an equilateral prism. The deviation produced by the prism does not depend on the:
(a) angle of incidence         (b) colour of light
(c) material of prism            (d) size of prism

Answer:

(d) size of prism

Explanation:

Angle of deviation depends on:

• Angle of incidence
• Refractive index
• Angle of the prism

Question 3
In the figure given below the correct statement is:

(a) ABED and ABC are the refracting surfaces.
(b) line AD is the refracting edge
(c) angle a is the angle of deviation
(d) AB is the base of the prism.

Answer:

(b) line AD is the refracting edge.

Explanation:

• The refracting edge is the edge where the two refracting surfaces of the prism meet — which in this triangle is AD.

• AB and AC are the refracting faces, and AD is the refracting edge.

Question 4
The angle of prism A is given as:
(a) \(r_1-r_2\)
(b) \(i_1+i_2\)
(c) \(r_1+r_2\)
(d) \(\frac{r_1}{r_2}\)

Answer:

(c) \(r_1+r_2\)

Explanation: 

The angle of a prism (A) is the angle between its two refracting surfaces, which equals the sum of the angles of refraction (\(r_1+r_2\)) inside the prism at minimum deviation.

Question 5
The angle of deviation becomes minimum when:
(a) \(i_1=r_1\)              (b) \(i_1=r_2\)
(c) \(i_1=i_2\)             (d) \(r_2=i_2\)

Answer:

(c) \(i_1=i_2\)

Explanation:

In the minimum deviation condition, the incident angle (i₁) is equal to the emergent angle (i₂), and the light ray passes symmetrically through the prism.

Question 6
In position of minimum deviation, the refracted ray inside the prism is …………… to its base:
(a) Normal
(b) inclined at 45°
(c) parallel
(d) none of the above

Answer:

(c) parallel

Explanation:

At minimum deviation, the internal ray is parallel to the base of the prism. This is a standard property of prisms in minimum deviation condition.

Question 7
Mention the incorrect statement:
(a) The angle of deviation produced by a prism depends on the colour or wavelength of the light used.
(b) The angle of incidence also affects the angle of deviation produced by the prism.
(c) In a prism as well as a parallel sided glass slab the emergent ray is parallel to the incident ray with a lateral displacement.
(d) The angle of deviation of prism is also affected by the material of the prism.

Answer:

(c) In a prism as well as a parallel sided glass slab the emergent ray is parallel to the incident ray with a lateral displacement.

Explanation:

Statement (c) is incorrect because, in a prism, the emergent ray is not parallel to the incident ray due to the non-parallel surfaces, unlike a parallel-sided glass slab.

Question 8
A ray of light is incident normally on the face of an equilateral glass prism. The angle of refraction from the first face of prism is :
(a) 45°                        (b) 90°
(c) 60°                        (d) 0°

Answer:

(d) 0°

Explanation: When a ray of light is incident normally (perpendicularly) on a surface, it means the angle of incidence is 0°.

Question 9
Assertion (A): If the angles of the base of a prism are equal, then in the position of minimum deviation, the refracted ray will pass parallel to the base of the prism.
Reason (R): For minimum deviation, angle of incidence is equal to the angle of emergence.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true and R is not the correct explanation of A
(c) assertion is false but reason is true
(d) assertion is true but reason is false

Answer:

(a) Both A and R are true and R is the correct explanation of A.

Explanation:

• Yes, in minimum deviation condition: \(i_1=i_2\), and the internal ray becomes parallel to the base.

• So both Assertion and Reason are correct, and Reason explains the Assertion.

Exercise 4(B): (B) Very Short Questions

Question 1
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 2
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:

False

Explanation:

The deviation does depend on the angle of incidence.

Question 3
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 4
Write a relation for the angle of deviation (𝛿) 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:

\(\delta=\left(i_1\ +\ i_2\right)-A\)

Question 5
Name the colour of white light which is deviated
(i) the most,
(ii) the least, on passing through a prism.

Answer:

(ii) Violet has the shortest wavelength, so it deviates the most.

(ii) Red has the longest wavelength, so it deviates the least.

Question 6
Which of the two prisms, A made of crown glass and B made of flint glass, deviates a ray of light more?

Answer:

Prism B (flint glass)

Explanation:

• Flint glass has a higher refractive index than crown glass.

• Hence, it produces greater deviation of light.

Exercise 4(B): (C)  Short Questions

Question 1
Define the term angle of deviation.

Answer:

The angle of deviation is the angle between the direction of the incident ray and the emergent ray after it passes through a prism.

Question 2
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) Greater the refractive index, greater is the deviation produced.

(ii) Shorter wavelengths (like violet) are deviated more, and longer wavelengths (like red) are deviated less.

Question 3
A ray of light incident at an angle of incidence  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.
(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) Angle of incidence i₁ is equal to the angle of emergence i₂
i.e., i₁= i₂

(ii) Minimum angle of deviation (denoted by δ_min).

Question 4
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:

Given,

Angle of incidence,  i₁ = 48°

Angle of minimum deviation, δ_min = 36°

Prism angle, A = 60° (since it’s an equilateral prism)

(i) In the case of minimum deviation i₁, the angle of incidence  is equal to the angle of emergence i₂.

Hence, angle of emergence,  i₂ = 48°

(ii) (a)  Angle of Incidence = 30°

This is less than 48°, which was the angle at minimum deviation.

When angle of incidence is less than the angle for minimum deviation, the deviation is greater.

The angle of deviation will be more than 36°.

Question 5
How does the angle of deviation depend on refracting angle of the prism?

Answer:

As the refracting angle (A) of the prism increases, the angle of deviation also increases.

Exercise 4(B): (D)  Long Questions

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.

Explanation of Parts:

1. Refracting Surfaces: The two inclined plane faces of the prism (AB and AC) through which light enters and exits the prism. These are responsible for bending the light rays.
2. Refracting Angle (A): The angle between the two refracting surfaces (∠BAC). It is also called the angle of the prism.
3. Base (BC): The opposite side of the prism to the refracting angle. It does not participate in refraction.

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)

(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) (a)

(b)

(ii) Difference in the emergent-ray direction

Prism	Glass slab
Emergent ray RS is not parallel to incident ray PQ; it is bent toward the base, making an angle δ (angle of deviation) with PQ.	Emergent ray RS is exactly parallel to PQ; there is no angular deviation, only a small lateral displacement .

Question 3
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:

Deviation (δ) is the angle between the directions of the incident ray and the emergent ray.

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

Three factors on which δ depends
The three factors on which the angle of deviation depends are as follows:
1. The angle of incidence (i)
2. The material of prism (i.e on refractive index μ)
3. The angle of prism (A)

Question 4
(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 𝛿_min is unique for light of a given wavelength.

Answer:

(a)

• When a ray of light is incident on a prism, the angle of deviation (δ) first decreases as the angle of incidence (i) increases.

• It reaches a minimum value, called the angle of minimum deviation (δ_min).
• After this point, as the angle of incidence increases further, the angle of deviation increases again.

(b) 

For a given prism and a specific colour of light, the angle of minimum deviation (𝛿ₘᵢₙ) is unique because only one horizontal line can be drawn parallel to the i-axis at the lowest point of the   i – 𝛿 curve. This indicates that there is only one specific angle of incidence (i) at which the refracted ray inside the prism travels parallel to its base.

Question 5
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:

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

Question 6
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 on the other side of the object.

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:

Question 7
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?
(c) What will be the angle of incidence at the second face of the prism?
(d) Will the light ray suffer minimum deviation by the prism?

Answer:

(a) Since the ray is normally incident, the angle between the incident ray and the normal 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°.

(c) Since the prism is equilateral (A = 60°), and the ray inside travelled straight through from the first face, the angle it makes with the second face from inside the prism is equal to the prism angle.
The angle of incidence at the second face of the prism is 60°.

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

Question 8
The diagram below shows two identical prisms A and B placed with their faces parallel to each other. A ray of light of single colour PQ is incident at the face of the prism A. Complete the diagram to show the path of the ray till it emerges out of the prism B.

[Hint: The emergent ray out of the prism B will be parallel to the incident ray PQ]

Answer:

The below diagram shows the complete path of the ray as it enters prism A and emerges out of prism B:

Exercise 4(B): (E)  Numericals

Question 1
A ray of light incident at an angle of incidence 48° on a prism of refracting angle 60° suffers minimum deviation. Calculate the angle of minimum deviation. [Hint: δ_min = 2i – A]

Answer:

Given,

Angle of incidence, \(i=48^\circ\)

Angle of prism, \(A=60^\circ\)

As we know,

\(\delta_{min}=2i-A\)

\(\Rightarrow\delta_{min}=\left(2\times48^\circ\right)-60^\circ\)

\(\Rightarrow\delta_{min}=96^\circ-60^\circ\)

\(\Rightarrow\delta_{min}=36^\circ\)

Hence, the angle of minimum deviation is equal to 36°

Question 2

What should be the angle of incidence for a ray of light which suffers minimum deviation of 36° through an equilateral prism?

Answer:

Given,

Angle of prism, \(A=60^\circ\)

Angle of minimum deviation, \(\delta_{min}=60^\circ\)

As we know,

\(\delta_{min}=2i-A\)

\(\Rightarrow36^\circ=\left(2\times i\right)-60^\circ\)

\(\Rightarrow i=\frac{36^\circ+60^\circ}2\)

\(\Rightarrow i=\frac{96^\circ}2=48^\circ\)

Hence, the angle of incidence is equal to 48°.

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(B) to revise offline. These solutions are exam-ready and designed by subject experts.

Key Benefits of Using Selina Solutions

• Step-by-Step Solutions – Each question is solved in a clear and logical sequence so you can follow along easily.

• Aligned with the ICSE Syllabus – Ensures that your preparation is in sync with board exam requirements.

• Covers Important Numericals – Many of these problems are similar to those asked in past board exams.

Topics Covered in Exercise 4(B)

• Application of refractive index formula
• Relationship between speed of light and refractive index
• Real-world examples like light passing through glass, water, or air
• Numerical problem-solving techniques for physics exams

Tips for Solving Exercise 4(B) Questions

• Always write down the given data before starting the solution.
• Use the correct formula for refractive index and rearrange it if needed.
• Keep track of units to avoid calculation errors.
• Cross-check your answer for reasonableness before finalizing it.