The effect in which an observed wave frequency changes because of relative motion is the Doppler effect.
As a source and observer move toward one another, successive wavefronts arrive closer together, so the observed frequency rises and the wavelength appears shorter. When they move apart, the wavefronts arrive more widely spaced, producing a lower observed frequency and a longer apparent wavelength. The wave source’s emitted frequency does not necessarily change; the observation changes because of motion.
Christian Doppler described the effect in 1842 while studying how motion could alter the apparent colour of stars. The familiar change in a passing ambulance siren is the acoustic version. Light also undergoes Doppler shifts: astronomers use redshift and blueshift to study the motion of stars, galaxies, and gas.
The effect is distinct from the Compton effect, which involves a change in photon wavelength after scattering, and from the photoelectric effect, which ejects electrons from materials.