Doppler Effect, an apparent change in the frequency of waves, due to the relative motion of the source of the waves and the observer. The classic example of the Doppler effect is the rising pitch of a train whistle as the train approaches an observer and the drop in pitch as it moves away from the observer. Not only sound waves, but also light waves, radio waves, and even ocean waves show the Doppler effect. The source of the waves may be either the original source, such as a star giving off light, or a secondary source, such as an object reflecting radar waves.

The explanation of the Doppler effect lies in the nature of waves. The frequency of a wave series is the number of waves that pass a given point each second, as measured at any particular part of the waves, such as the crests. When the observer is moving toward the source (or the source toward the observer), the observer perceives the waves passing at a higher frequency than he or she would otherwise. When the observer is moving away from the source (or the source from the observer), the observer perceives the waves passing at a reduced frequency. Any characteristic of the waves dependent upon frequency, such as the pitch of sound or the color of visible light, is therefore perceived as changed.

The Doppler effect was described in 1842 by Christian J. Doppler, an Austrian physicist. Although it was originally proposed to describe sounds, it was soon extended to all forms of waves.

Astronomers discovered that, combined with the identification of spectra, the Doppler effect could be used to determine whether stars and galaxies were traveling away from or toward the earth. When a star is traveling away from the earth, the lines in the spectra of the elements in the star are shifted toward the red end of the visible spectrum. Celestial objects that are approaching the earth have spectra shifted toward the violet. The amount of shift in either direction is related to the velocity of the object with respect to the earth. The red shift shown by all galaxies outside the Milky Way is the principal argument in favor of the theory that the entire universe is expanding.

The Doppler effect is the basis for radar equipment that determines how fast aircraft, automobiles, and weather systems are moving. A radar signal of known frequency is bounced off the moving object and the frequency of the echo is measured; the speed of the object is calculated from the difference in the frequencies.