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In Young’s double slit… (Class 12 Physics SQP 2026-27, Q15)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 1 mark · Assertion-reason

CBSE Class 12 Physics sample paper 2026-27: question 15

From the official CBSE sample question paper for 2026-27, section A, question 15, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q15 · 1 mark · Assertion-reasonCompetency-basedWave Optics

In Young’s double slit experiment, the distance between the slits is 0.25mm and the screen is placed 1.0m away. A student uses monochromatic light of wavelength 500nm.

Assertion (A): The fringe width observed on the screen will be 2.0mm.

Reason (R): The fringe width in YDSE is directly proportional to the slit separation and inversely proportional to the wavelength of light.

  1. (A)both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  2. (B)both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
  3. (C)Assertion is true but Reason is false.
  4. (D)both Assertion and Reason are false.

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

Use with SI units.

Hint 2

Check how depends on and 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.

  1. Fringe width: m mm, so A is true.

    ½ mark

  2. From the formula, and . R states the reverse, so R is false. Option (C).

    ½ mark

Answer: Option (C)

Where the marks are

  • Computes mm and notes that R reverses the proportionality (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 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.