Skip to content

A diamond has a… (Class 12 Physics SQP 2026-27, Q25)

questionbuddy.in/q/cbse-12-physics-2026-27-sqp-q25-diamond-refractive-index-ray-light

PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2026-27: question 25

From the official CBSE sample question paper for 2026-27, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerRay Optics and Optical Instruments

A diamond has a refractive index of 2.42.

I. A ray of light inside the diamond strikes the surface at an angle of incidence of . State whether it will undergo refraction or total internal reflection.
II. Explain why diamonds sparkle when cut properly.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Find the critical angle using sin θc = 1/μ.

Hint 2

Compare 30° with θc for light going from diamond to air.

Hint 3

What happens to light repeatedly hitting facets at angles larger than θc?

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.

  1. Critical angle for diamond–air: .

    1 mark

  2. I. The ray travels from denser (diamond) to rarer (air) medium and , so it undergoes total internal reflection, not refraction.

    1 mark

  3. II. Because diamond's critical angle is very small, a properly cut diamond has facets angled so that light entering it strikes the inner faces at angles greater than and is totally internally reflected many times. Light finally leaves through only a few faces, concentrated and dispersed into colours, so the diamond sparkles.

    1 mark

Answer: θc ≈ 24.4°; I. total internal reflection; II. repeated TIR due to small critical angle and proper cut.

Where the marks are

  • Calculates critical angle ≈ 24.4° (1 mark)
  • Concludes TIR since 30° > θc (1 mark)
  • Explains sparkle by multiple TIR from cut facets (1 mark)

Revise the chapter

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 paperSample paper 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.