MODEL RELEASED. Radar speed gun. A policewoman shows the speed display on a radar speed gun to a driver who has broken the speed limit. When this gun is pointed at a moving object it displays the speed in kilometres per hour on a small screen on the back of the gun. The device works by bouncing a radar beam off the vehicle and comparing the frequency of the reflected signal to that of the initial output. Due to the doppler effect, the reflected frequency is lowered by a factor that depends on the speed of the moving object. The same effect is responsible for the sudden drop in the pitch of a siren when an ambulance passes a stationary observer.