the Doppler effect

The Doppler effect is the change in frequency or wavelength of a wave as the source and observer move relative to each other. When a sound source approaches, the sound waves are compressed, resulting in a higher pitch. Conversely, as the source moves away, the waves are stretched, leading to a lower pitch. This phenomenon applies to all types of waves, including light; for example, galaxies moving away from us appear redshifted. The Doppler effect is commonly experienced with passing trains or emergency vehicles, where the pitch of the siren changes as they approach and then recede.