How Does a Radar Gun Measure Speed?
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Radar
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Radar is a system which uses radio waves to find objects which are both stationary and in motion. The word radar is an abbreviation of "radio detection and ranging." Like many technologies, radar systems used to be huge, taking up entire rooms which made them only practical for use at airfields or installed on large warships. However, as time went on and technology progressed, radar systems have become smaller and smaller. Radar guns, most often used by police to track the speed of vehicles driving towards them on the highway, are an example of just how compact radar technology has truly become.
Process
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How a radar gun works is very similar to how laser surveying devices function. By pulling the trigger on a radar gun, a cone of radio waves are sent out from the gun's "barrel," similar to how a flashlight would shine out. The radio waves travel through the air, and reflect off of whatever objects they hit. When they hit a moving object, such as a speeding car, the waves return to the radar gun's sensors at a higher pitch than they were at when they were fired. By calculating the difference in the pitch, the radar gun can come up with the vehicle's approximate speed.
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The Doppler Effect
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Named for the first person who put words to the theory, Christian Andreas Doppler, the Doppler Effect is the reason that radar guns can function. What the Doppler Effect states is that radio waves that reflect from objects moving towards a person will have a higher pitch than radio waves that reflect from objects moving away from a person. Using the Doppler Effect, the computer attached to a radar gun can determine a vehicle's speed regardless of whether it's moving towards or away from the gun.
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