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How does police radar work?

Police radar, which detects the speed of passing vehicles and displays that speed on a screen, operates on the principle of the Doppler effect. The Doppler effect is the change in frequency (number per unit time) of sound or light waves emitted from a moving source. The Doppler effect was explained by Austrian physicist Christian Doppler (1803-1853) in 1842. According to Doppler, waves bunch up as they approach their target and spread out as they move away from their target.

Radar (an acronym for "radio detection and ranging") is a device that emits and receives radio waves. The waves bounce off the targeted vehicle and are received by a recorder. The recorder compares the difference between the sent and received waves, and translates the information into miles per hour.

Sources: Bogner, Bruce F. Vehicular Traffic Radar: Handbook for Attorneys, p. 4; Schneider, Herman. The...

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