The “Doppler effect” is a phenomenon observed when there’s a change in frequency or wavelength of a wave in relation to an observer’s motion relative to the source of the wave. This effect commonly applies to sound or light waves but is applicable to any type of wave. In the case of sound, if a source of sound and an observer are in motion relative to each other, the perceived frequency of the sound changes. For instance, as a moving vehicle emitting a siren approaches an observer, the sound waves get compressed, causing the pitch to increase (higher frequency). Conversely, as the vehicle moves away, the sound waves stretch out, resulting in a lower frequency and a perceived drop in pitch.