What is a 'Doppler shift'?

A 'Doppler shift' refers to the phenomenon where there is a change in frequency or wavelength of waves as the source of the waves moves relative to an observer. This effect is commonly experienced in sound waves, such as the changing pitch of a vehicle's siren as it approaches and then moves away from an observer. In the context of radar and lidar technology, the Doppler shift is utilized to determine the speed of an object by measuring the change in frequency of the reflected waves when the source of those waves changes position. This concept is crucial in speed detection and helps law enforcement accurately monitor vehicle speeds.