7.11 Doppler Effect

7.11 Doppler Effect

The apparent change in the frequency (or pitch) of waves due to the relative motion between the source and observer (listener) is called Doppler Effect.

This effect was first observed by John Doppler while he was observing the frequency of light emitted from a star. In some cases, the frequency of emitted light was found to be slightly different from that emitted from a similar source on the Earth. He found that the change of frequency of light depends upon motion of star relative to Earth.

This effect can be observed with sound waves also. For example, when an observer is standing on a railway platform, the pitch of whistle of an engine coming towards the platform appears to become higher to an observer standing on the platform. However, the pitch of whistle of an engine going away from the platform appears to become lower to an observer standing on the platform.

Consider a source of sound $S$ at rest which emits sound waves having wavelength $\lambda$. Let speed of the sound for a stationary observer (i.e., listener) is $v$ then the number of waves received by observer in one second i.e., frequency $f$ is: