How Stuff Works

Radar guns use radio waves to measure the speed of moving objects. After emitting a pulse, the gun tracks the amount of time the radio wave takes to echo off an object and its change in frequency--also known as its Doppler shift. The gun then computes how fast the object is moving in miles per hour.

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Moving at the speed of light, these electromagnetic waves are invisible to the human eye. The frequency of a radio wave--the number of times it passes a fixed point in a second--determines its strength. Air traffic control radar, for example, uses frequencies of 960 to 1,215 megahertz, whereas FM radio uses frequencies of 88 to 108 megahertz. Because they can be detected at a distance, radio waves often are used to determine the locaton of faraway objects.

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Any offset between a stationary radar gun and a moving target creates a “cosine effect” that reduces the return frequency by the cosine of the angle between the gun and the vehicle, thus lowering the speed reading. Therefore, police try to park close to the road and detect vehicles far downfield.

Researched by CHRISTINE FREY and MARK HAFER/Los Angeles Times

Source: www.howstuffworks.com