Radar detection of car speed

Doppler effect

Fig. 1  Doppler effect

You must have experienced this: When an ambulance is steered towards you with its alarm on, you will hear the sound with a higher pitch; but when it leaves you, the pitch becomes lower. This is resulted from Doppler effect. When there is a relative motion between a wave source and a receiver, the latter will detect a change in the wave frequency. Take sound wave as an example. If a wave source emitting a sound wave with a frequency and a wavelength is moving towards a receiver, and the receiver stays still, the receiver will hear a sound wave with a frequency larger than , and a wavelength smaller than , that is, a higher pitch (Fig. 1).

Radar used by police is based on this mechanism. They have an emitter and a receiver of radar. Firstly, the emitter will emit electromagnetic wave with a frequency towards a vehicle. When the electromagnetic wave is reflected by the vehicle, the vehicle acts like a wave source moving towards the police, emitting an electromagnetic wave at a frequency slightly larger than . The receiver will receive and analyze the reflected electromagnetic wave and work out the speed of the vehicle. I have seen a policeman hid aside of the highway using radar to detect driving speed, and there is a road blockade right at the front of the highway! Once they discover anyone driving over-speed, they will inform their colleagues stationing at the blockade to stop the vehicle and then fine the driver!