Illustration of the doppler effect. This describes the change in frequency of a wave for an observer moving relative to its source. The illustration uses the sound of an ambulance as an example. Compared to the emitted frequency (represented at left), the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession (represented at right). 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 named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.