A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency ( n ′ ) of the sound heard by the ob

Question

A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency ( n ′ ) of the sound heard by the observer is plotted against time ( t ) , which of the following best represents the resulting curve

TestHub · Accepted Answer

It is known that Doppler's effect depends on velocity not on distance. When the source is approaching the stationary observer, the apparent frequency heard by the observer is

$n^{'} = \frac{v \times n}{v - v_{s}} = c o n s t a n t$

But $n^{'} > n$ .

When the source has crossed the observer, apparent frequency heard by the observer is

$n^{''} = \frac{v \times n}{v + v_{s}} = a n o t h e r c o n s t a n t$

But $n^{''} < n$ .option (d) is correct.

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