Skip to content

Monochromatic light of… (Class 12 Physics 2024, Q18a)

questionbuddy.in/q/cbse-12-physics-2024-55-1-1-q18a-monochromatic-light-frequency-passes-air

PhysicsCBSE Class 12 · Board exam 2024, set 55/1/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2024: question 18(a)

Set in the CBSE Class 12 board exam of 2024, paper 55/1/1, section B, question 18(a), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18(a) · 2 marks · Very short answerWave Optics

Monochromatic light of frequency Hz passes from air into a medium of refractive index 1.5. Find the wavelength of the light (i) reflected, and (ii) refracted at the interface of the two media.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 55/1/1.

Hints

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

Hint 1

Which property of light stays the same when it is reflected or refracted?

Hint 2

Reflected light travels in air, so use .

Hint 3

In the medium the speed becomes while the frequency is unchanged, so the wavelength changes by the same factor.

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. (i) The reflected light stays in air, so its speed is and the frequency is unchanged. Using : m = 600 nm.

    1 mark

  2. (ii) On refraction the frequency stays the same but the speed falls to , so nm m.

    1 mark

Answer: (i) 600 nm; (ii) 400 nm

Where the marks are

  • Uses to get the reflected wavelength 600 nm (1 mark)
  • Uses to get the refracted wavelength 400 nm (1 mark)

Revise the chapter

  • Chapter: Wave Optics (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Wave Optics, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 55/1/1: 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.