Question

A man sitting in a moving train hears the whistle of the engine. The frequency of the whistle is 600Hz.
A. The apparent frequency as heard by him is smaller than 600Hz
B. The apparent frequency is larger than 600Hz.
C. The frequency as heard by him is 600Hz.
D. None of them above.

Answer 1

As a first step, one could read the question well along with the options. Now you may recall that the question is related to the doppler effect as there is a mention of apparent frequency. However, recall the definition of doppler effect to find the frequency heard by the observer.

Complete step by step answer:
In the question, we are told that a man is sitting in a moving train and this man hears the whistle of the engine. We are also told that the frequency of the whistle is 600Hz. We are supposed to find which of the given options are true for the given situation. In order to answer this question, let us recall the concept of the doppler effect.
One may recall that the doppler shift or the doppler effect is the term given to the change in frequency of a wave observed as the result of relative motion between the observer and the source of the wave. The word to be highlighted here is the ‘relative motion’. So, in order for the doppler shift or the change in frequency to be observed, there should definitely be a relative motion between the source and observer.
In the given question, there exists no relative motion between the source and the observer. Hence, the frequency as heard by him would be the frequency of the whistle which is 600Hz.

So, the correct answer is “Option C”.

Note: Now, in case if you wonder how a doppler shift happens, the reason lies behind the fact that each wave takes less time to approach the observer than the previous one due to the source approaching the observer. And for the source going away from the observer , arrival time between successive waves is increased.