Answer: B. Doppler Effect
Explanation: Also known as Doppler shift, from Johann Christian Andreas Doppler, the scientist who first proposed this effect, the Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. As such, the
Doppler radar such as those employed by the police uses reflected microwaves to determine the speed of moving objects which it does this by emitting waves of a particular frequency, and then analysing the reflected wave for frequency changes.
