What is the Doppler effect?

Have you heard of the "Doppler effect" ?
The change in sound of a siren on an ambulance as it speeds past is one familiar example.
The siren on an ambulance is emitted at a constant pitch.
However, to a stationary person, the siren can be heard at a high pitch as the ambulance approaches, and a low pitch as it moves away. This phenomenon occurs according to change in distance between the object emitting sound and the object receiving sound. This is called the Doppler effect.

The Doppler effect does not only occur with sound, but also with radio waves, which are the same kind of wave.
By measuring changes with the Doppler effect, velocity can be determined.