In some countries, police use a Doppler radar gun to measure the speed of cars travelling along the road.
(a) Outline how the police are able to get an accurate value for the speed of a moving car.
(b) A typical police Doppler radar gun uses electromagnetic waves of frequency 24 GHz.
i In which part of the electromagnetic spectrum do such waves occur?
ii If a police officer measures a frequency change of 2.0 kHz when aiming the radar gun at a receding car, calculate the speed of the car.
(c) Suggest why Doppler radar guns do not use ultrasonic waves instead of microwaves.