Assertion (A) : A ray of light is incident normally on the face of a prism. The emergent ray will graze along the opposite face of the prism when the critical angle at glass-air interface is equal to the angle of the prism. Reason (R) : The refractive index of a prism depends on angle of the prism. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

PhysicsCBSE Class 12 · Board exam 2025, set 55/7/1 · 1 mark · Assertion-reason

CBSE Class 12 Physics 2025: question 14

Set in the CBSE Class 12 board exam of 2025, paper 55/7/1, section A, question 14, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q14 · 1 mark · Assertion-reasonCompetency-basedRay Optics and Optical Instruments

Assertion (A) : A ray of light is incident normally on the face of a prism. The emergent ray will graze along the opposite face of the prism when the critical angle at glass-air interface is equal to the angle of the prism.
Reason (R) : The refractive index of a prism depends on angle of the prism.

(A)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
(B)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
(C)Assertion (A) is true, but Reason (R) is false.
(D)Both Assertion (A) and Reason (R) are false.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 55/7/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Use $r_{1} + r_{2} = A$ .

Hint 2

What property of glass does μ actually depend on?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

At normal incidence $r_{1} = 0$ , so $r_{2} = A$ . If $A = C$ , the emergent ray grazes the second face, so A is true.
½ mark
The refractive index depends on the material and the wavelength of light, not on the prism angle, so R is false.
½ mark

Answer: Option (C)

Where the marks are

Uses $r_{1} + r_{2} = A$ with $r_{1} = 0$ and rejects R (1 mark)

Revise the chapter

Chapter: Ray Optics and Optical Instruments (notes, formulas, where marks are lost and every past question)
Topic: Total Internal Reflection

Practise this chapterBoard-style questions from Ray Optics and Optical Instruments, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 55/7/1: every question, by section.

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CBSE Class 12 Physics 2025: question 14

Hints

Step-by-step solution

Revise the chapter

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