A train whistling at constant frequency is moving towards a station at a constant speed V. The train goes past a stationary observer on the station. The frequency `n^(')` of the sound as heard by the observer is plotted as a function of time t, figure. Identify the expected curve.

Options

A.
B.
C.
D.

Answer

C

Explanation

This is based on Doppler effect. The observer is stationary on the platform in the station. Hence when the train approaches the observer, the apparent frequency `n_(a) gt` real frequency of the whistle. After crossing the observer, the train goes away from the observer.
Hence the apparent frequency decreases. `n_(a) lt n`
This is shown in fig (c)