Let a stationary observer on a platform listen to the sound emitted by the whistle of an incoming train. As the train approaches the platform, an increase in the pitch of the sound will be observed. As the engine moves away from him, there will be a decrease in the pitch of the sound. The same effect is observed in the case of approaching or receding automobiles and jet aircrafts flying at a low altitude. This effect is also felt when the observer approaches or moves away from a stationary source. In these cases, it must be noted that the source is emitting sound of a particular frequency, but it is the observed frequency that changes. This phenomenon in which there is an apparent change in the frequency of sound as a result of relative motion between the source and the observer is called Doppler effect.
The Doppler effect or Doppler shift was first discussed by Christian Johann Doppler (1803 - 1853) in 1842, in connection with similar shifts in the frequency of light emitted by the stars revolving about each other in double-star systems. It was tested experimentally, in connection with sound, in 1845, by Berys Ballot in Holland.
