The Doppler effect is used in police radar to measure what?
Answer
Speed of vehicles
The Doppler effect is used in police radar to measure the speed of vehicles.
A radar speed gun sends radio waves toward a moving vehicle and receives the reflected signal. If the vehicle approaches, the returning waves have a higher frequency; if it moves away, they have a lower frequency. The size of this frequency shift reveals the vehicle’s speed along the radar beam.
The device does not directly measure distance or acceleration. Radar can determine distance using the signal’s travel time, but a police speed gun primarily relies on Doppler frequency change for velocity. It also measures only the component of motion toward or away from the instrument, so the angle between the radar and road matters.
The same principle is used outside traffic enforcement. Doppler radar measures wind, weather movement, and the speeds of baseball pitches, tennis serves, and cricket deliveries. Police may also use lidar speed guns, which measure speed with laser pulses rather than radio waves.