The radar used by police to check over-speeding vehicles works on the principle of

Police radar guns used for speed enforcement operate on the principle of the Doppler effect [1]. This phenomenon involves a change in the frequency or wavelength of a wave relative to an observer moving in relation to the wave source. When a radar gun emits radio waves toward a moving vehicle, the waves reflect off the vehicle and return to the receiver [1]. If the vehicle is moving toward the radar, the frequency of the reflected signal increases; if it is moving away, the frequency decreases. This frequency shift, known as the Doppler shift, is directly proportional to the vehicle's speed. By analyzing this shift between the emitted and received signals, the device accurately calculates the target's velocity. Other options like the Raman effect (spectroscopy), Induction effect (magnetism), and Coulomb effect (electrostatics) do not govern radar speed detection.

Sources

1. [1] https://en.wikipedia.org/wiki/Radar_speed_gun