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DOPPLER EFFECT IN LIGHT

Whenever there is a relative motion between the source of sound, the observer and the medium; the frequency of sound as received by the observer is different from the frequency of sound emitted by the source. The apparent change in the frequency of sound when the source, the observer and the medium are in relative motion is called doppler effect, For example , consider a man standing on a railway platform. When a trains blowing its whistle appears to rise and it suddenly appears to drop as the engine moves away from him. Similar effect is observed when the source is at rest and the observer moves towards or away from the source.

Doppler Effect is a wave phenomenon, it holds not only for sound waves but also for electromagnetic waves such as microwaves, radio waves and visible light. However, Doppler effect is noticeable only when the relative velocity between the source and the observer is an appreciable fraction of the wave velocity.

For the waves, which require a medium for their propogation, the apparent frequency depends on three factors.

(I) Velocity of the source,

(ii) Velocity of the observer and

(iii) Velocity of medium or wind.