What's The Doppler Effect?

The Doppler Effect is the change in frequency of a wave (sound or light) due to the motion of the source or observer. The frequency (and hence pitch) of a sound appears to be higher when the source approaches the listener and lower when the source recedes from him.

It is due to the Doppler Effect that the whistle of a train appears shriller when it approaches a listener than when it moves away from him.

The Doppler Effect is very useful in Astronomy. It can be used to find out whether a star is approaching us or receding away from us. When a star is receding away from us, the light emitted from the star appears red (red light is of lower frequency than other colours). Thus the fact that the light emitted by the stars of distant galaxies suffers a red shift when observed from the earth, means that these galaxies are receding from our galaxy. This is the principle evidence in favour of the hypothesis of expanding universe.

Doppler Effect can also be used to detect or even measure the rotation of a star, e.g. the sun.

Doppler effect is defined as the apparent change in the pitch of sound of the source caused by the relative motion between the source and the observer.When the source of sound approaches the observer at rest, the effect is shortening of the wavelength as the source is following after the waves approaches the observer at rest. As a result, frequency of sound apparently increases. When the observer moves towards the stationary source, the pitch of sound apparently increases. If both the observer and source moves away from each other, the frequency of the sound waves decreases. On the other hand, if both the observer and source moves towards each other, frequency apparently increases.