A man is standing on a railway platform listening to the whistle of an engine, that passes the man at constant speed without stopping. If the engine passes the man at time `t_(0)`, how does the frequency f of the whistle as head by the man changes with time ?

Explanation

According to Dopper's effect in sound, the frequency of sound heard dows not vary with time. If varies only with the speed of source `//` listener and the direction of their motion. Upto time `t_(0)`, the source is moving towards the man. Frequency heard is more than the actual frequency emitted. After time `t_(0)`, source is moving away from the man. The frequency heard is less than the actual frequency. Hence choice (C) is correct.