The Doppler effect is the change in observed frequency caused by relative motion between a wave source and an observer.

Moving source or observer

If source and observer move toward one another, wavefronts arrive more frequently and the observed frequency increases. If they move apart, the observed frequency decreases.

For sound in a stationary medium, a common one-dimensional form is

$$f'=f\frac{v+v_o}{v-v_s},$$

where $v$ is wave speed in the medium and the signs of observer speed $v_o$ and source speed $v_s$ are chosen consistently with motion toward one another.

Why source and observer motion differ

A moving observer encounters existing wavefronts at a different rate. A moving source changes the spacing of the wavefronts emitted into the medium, and therefore changes their wavelength.

Applications

The effect appears in sound, electromagnetic radiation and other waves, although the relativistic formula for light differs from the classical sound formula.

Doppler measurements can infer relative motion from frequency shifts, as in radar, medical ultrasound and astronomical redshift or blueshift.