Have you ever noticed that when an ambulance with its siren blaring speeds by, the pitch of its sound changes from high to low? This is the famous Doppler effect. It describes the phenomenon that the frequency of a wave received by an observer changes when there is relative motion between the wave source and the observer. When the wave source (such as a sound source or light source) moves towards the observer, the wavefront is compressed, the wavelength becomes shorter, and the frequency received by the observer becomes higher (e.g., the pitch becomes sharper); conversely, when the wave source moves away, the wavefront is stretched, the wavelength becomes longer, and the frequency received becomes lower (e.g., the pitch becomes deeper). This principle applies not only to sound waves but also to light waves (causing redshift and blueshift), water waves, and all other wave phenomena. It is the core physical foundation for astronomical measurements of stellar velocities, radar velocimetry
