Source S emits microwaves with a constant amplitude. The microwaves hit a metal screen P and are reflected.

Question 12

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.

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?

A decreases steadily

B increases and decreases regularly

C increases steadily

D remains constant

Reference: Past Exam Paper – June 2015 Paper 12 Q27

Solution:

Answer: B.

Since a stationary wave is formed, there would be nodes (points with minimum amplitude) and antinodes (points with maximum amplitude).

We are told that the wavelength of the microwaves is much smaller than the distance between S and P. This means that the stationary wave consists of many wavelengths and thus, many nodes (N) and antinodes (A).

So, the stationary wave consists of many nodes and antinodes such as:

N A N A N A N A ……. N

 

The amplitude of the signal detected by Q would increase (at antinodes) and decrease (at nodes) regularly.