Doppler effect
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The Doppler effect is the change in frequency of a wave that an observer perceives when either the observer or the source of the wave is moving. This matters because it explains everyday experiences like why a siren sounds higher-pitched as an ambulance approaches and lower-pitched as it moves away, and it's also used in practical applications like radar speed guns and astronomy.
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Change of wavelength caused by motion of the source An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The red circles represent sound waves.
The Doppler effect (also Doppler shift) is the change in the frequency or, equivalently, the period of a wave in relation to an observer who is moving relative to the source of the wave. It is named after the physicist Christian Doppler, who described the phenomenon in 1842. A common example of Doppler shift is the change of pitch heard when a vehicle approaches and recedes from an observer. Compared to the emitted sound, the received sound has a higher pitch during the approach, identical at the instant of passing by, and lower pitch during the recession.