What principle does radar speed detection rely on?

Radar speed detection primarily relies on the Doppler effect. This principle describes how the frequency of waves changes in relation to an observer moving relative to the source of the waves. When a radar unit emits radio waves towards a moving vehicle, these waves reflect back to the radar unit. If the vehicle is moving toward the radar, the frequency of the returned waves is higher than the emitted waves; conversely, if the vehicle is moving away, the frequency is lower.

By measuring this change in frequency, the radar system can calculate the speed of the vehicle. This application of the Doppler effect is critical, as it allows law enforcement and traffic monitoring entities to accurately assess vehicle speed in real-time, making it an essential principle in radar technology.

The other principles listed, while relevant in the broader context of wave physics or radar technology, do not specifically characterize the core operation of radar speed detection as effectively as the Doppler effect does.