DOPPLEREFFECT
Doppler effect
The Doppler effect, named after the 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 its source. 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.The above text is a snippet from Wikipedia: Doppler effect
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