The Doppler effect and Doppler shift were named after the Austrian physicist and mathematician Christian Johann Doppler in 1842, who conducted experiments with both moving sources and moving observers. Consider an observer standing on a street corner, observing an ambulance with a siren sound passing by at a constant speed. The observer experiences two characteristic changes in the sound of the siren. Initially, the sound increases in loudness as the ambulance approaches and decreases in loudness as it moves away, which is expected. But, in addition, the high-pitched siren shifts dramatically to a lower-pitched sound. As the ambulance passes by, the frequency of the sound heard by the stationary observer changes from a constant high frequency to a constant lower frequency, even though the siren is producing sound at a constant source frequency. This shift in the frequency and loudness of the sound wave heard by the observer is known as the Doppler effect.

The Doppler effect occurs not only for sound waves but for any wave when there is relative motion between the observer and the source. A Doppler shift occurs in the frequency of sound, light, and water waves, for instance. A Doppler shift can be used to determine velocity, such as when an ultrasound is reflected from blood in a medical diagnostic. Furthermore, the relative velocities of the stars and galaxies are determined by the shift in the frequencies of light waves received from themand have implied much about the origins of the universe. Modern physics has been profoundly affected by observations of Doppler shifts.

This text is adapted from Openstax, University Physics Volume 1, Section 17.7: The Doppler Effect.