The
Doppler effect, named after
Austrian physicist
Christian Doppler who proposed it in 1842 in
Prague, is the change in
frequency of a
wave for an
observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a
siren or horn approaches, passes, and recedes from an observer. The received frequency is higher during the approach, it is identical at the instant of passing by, and it is lower during the recession.