Basic Functioning

• Radar guns work by sending out a radio signal. This signal hits the target and bounces back to the gun's receiver. However, when the signal returns to the radar gun, the frequency has been shifted, and the degree to which the signal has shifted can be analyzed to determine the speed of the target. This frequency shift and its analysis rely on the Doppler effect.
  
  

The Doppler Effect

• The Doppler effect, also sometimes called the Doppler shift, describes the change in frequency or wavelength for observers relative to the origin of the transmission. It applies to most anything that travels in wave form, and is most commonly observed when an ambulance or police car blaring its siren passes by. The shift in the noise of the siren as the vehicle moves is due to the Doppler effect. For radar guns, the only real factor for the Doppler effect is the relative difference in speed between the radar transmitter and the target.
  
  

Practical Problems

• Radar guns do have their limitations. The biggest one is that to get an accurate reading the target should be the only moving target in the radar beam's path. Achieving that on a busy road is difficult, and requires some specialized training. Other sources of interference include the angle of the target relative to the radar gun, and other transmitters like cell phones and CB radios.