Source S emits microwaves with a constant amplitude. The microwaves hit a metal screen P and are reflected. A stationary wave is formed between S and P. The wavelength of the microwaves is much smaller than the distance between S and P. [Figure showing source S, detector Q, and metal screen P. Q moves from S to P.]
A detector Q is moved at a slow, constant speed from S to P.
What happens to the amplitude of the signal detected by Q?

Q: What topic does this question test?

This question tests Superposition in A-Level Physics (syllabus code 9702). It is worth 1 mark.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Physics May/June 2015 examination, Paper 1 Variant 2 (9702_s15_qp_12.pdf).

Question

Source S emits microwaves with a constant amplitude. The microwaves hit a metal screen P and are reflected. A stationary wave is formed between S and P. The wavelength of the microwaves is much smaller than the distance between S and P. [Figure showing source S, detector Q, and metal screen P. Q moves from S to P.]
A detector Q is moved at a slow, constant speed from S to P.
What happens to the amplitude of the signal detected by Q?

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The correct answer is B: increases and decreases regularly.

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