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Write two differences in the… (Class 12 Physics 2025, Q19)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/7/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2025: question 19

Set in the CBSE Class 12 board exam of 2025, paper 55/7/1, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q19 · 2 marks · Very short answerWave Optics

Write two differences in the patterns of double-slit interference experiment and single-slit diffraction experiment.
Light waves from two pinholes illuminated by two sodium lamps do not produce interference patterns. Explain why.

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

Compare the widths and intensities of the bright bands in the two patterns.

Hint 2

What condition must two sources satisfy for a steady interference pattern?

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. Difference 1: The double-slit interference pattern has many equally spaced bright and dark fringes of equal width. In the single-slit diffraction pattern, the central bright maximum is twice as wide as the other maxima.

    ½ mark

  2. Difference 2: In interference, all bright fringes have (nearly) the same intensity. In diffraction, the intensity of successive maxima falls rapidly on either side of the centre. (Another valid difference: interference results from superposing two waves from two slits, while diffraction results from superposing a continuous family of wavelets from different points of a single slit.)

    ½ mark

  3. Two separate sodium lamps are independent, incoherent sources. Each emits light in random bursts, so the phase difference between the waves from the two pinholes changes rapidly and randomly with time.

    ½ mark

  4. The positions of maxima and minima therefore shift too quickly for the eye to follow. Only uniform average illumination is seen, and no sustained interference pattern forms.

    ½ mark

Answer: Interference: equally spaced, equal-width, equal-intensity fringes. Diffraction: central maximum twice as wide, with secondary maxima of decreasing intensity. The sodium lamps are incoherent (no constant phase difference), so no steady pattern forms.

Where the marks are

  • Any two correct differences between the patterns (½ + ½ mark)
  • States that the two lamps are incoherent sources (½ mark)
  • Explains that the phase difference is not constant, so no sustained pattern forms (½ 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 paper2025 board paper, set 55/7/1: 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.