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Assertion (A): In a Young's… (Class 12 Physics 2024, Q15)

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PhysicsCBSE Class 12 · Board exam 2024, set 55/5/1 · 1 mark · Assertion-reason

CBSE Class 12 Physics 2024: question 15

Set in the CBSE Class 12 board exam of 2024, paper 55/5/1, 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

Assertion (A): In a Young's double-slit experiment, interference pattern is not observed when two coherent sources are infinitely close to each other.

Reason (R): The fringe width is proportional to the separation between the two sources.

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

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 55/5/1.

Hints

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

Hint 1

Write the fringe width formula.

Hint 2

Check how β depends on d.

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. ; as , fringe width becomes very large, so a single fringe fills the screen and no pattern is seen. Assertion true.

    ½ mark

  2. Fringe width is inversely (not directly) proportional to , so Reason is false. Option (C).

    ½ mark

Answer: Option (C)

Where the marks are

  • Assertion true, Reason false since β ∝ 1/d (1 mark)

Revise the chapter

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 55/5/1: 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.