Assertion (A): de Broglie's wavelength of a freely falling body keeps decreasing with time. Reason (R): The momentum of the freely falling body increases with time. (A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion. (B) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion. (C) If Assertion is true but Reason is false. (D) If both Assertion and Reason are false.

PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 1 mark · Assertion-reason

CBSE Class 12 Physics sample paper 2024-25: question 16

From the official CBSE sample question paper for 2024-25, section A, question 16, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q16 · 1 mark · Assertion-reasonCompetency-basedDual Nature of Radiation and Matter

Assertion (A): de Broglie's wavelength of a freely falling body keeps decreasing with time.
Reason (R): The momentum of the freely falling body increases with time.

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

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Write the de Broglie relation.

Hint 2

How does the speed of a falling body change with time?

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.

For a falling body $v = g t$ , so its momentum $p = m g t$ increases with time. Since $λ = h / p$ , the wavelength decreases.
½ mark
Both statements are true and R explains A. The answer is (A).
½ mark

Answer: Option (A)

Where the marks are

$λ = h / p$ with p increasing (1 mark)

Revise the chapter

Chapter: Dual Nature of Radiation and Matter (notes, formulas, where marks are lost and every past question)
Topic: de Broglie Wavelength

Practise this chapterBoard-style questions from Dual Nature of Radiation and Matter, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

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CBSE Class 12 Physics sample paper 2024-25: question 16

Hints

Step-by-step solution

Revise the chapter

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