The Doppler effect applies to what type of wave?

The Doppler effect applies to all waves. It describes a change in the frequency, or equivalently the period and wavelength, measured by an observer when the source and observer move relative to one another.

The familiar example is an ambulance siren: its pitch sounds higher as the vehicle approaches and lower as it moves away. The same principle applies to sound, visible light, radio waves, water waves, and seismic waves. The details differ because waves travel through different media—or, in the case of light, through vacuum—but the underlying frequency shift is the same.

A common mix-up is to treat the Doppler effect as a sound-only phenomenon. Sound provides the easiest everyday example, while astronomers use Doppler shifts in light to measure whether stars and galaxies are approaching or receding. Radar systems also use the effect to measure the motion of rain, vehicles, and aircraft.