Definition for View source for Doppler effect

The Doppler effect (or Doppler shift) is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.<br>

http://upload.wikimedia.org/wikipedia/commons/9/90/Dopplerfrequenz.gif<br>