When a source of sound moves toward or away from a listener, the pitch, or frequency of the sound is higher or lower than when the source is at rest. A common example of this effect is the rise and fall of the pitch of the whistle of a locomotive as it approaches and recedes from the listener. Similar results are obtained when the listener approaches or recedes from a stationary source of sound. This phenomenon, the Doppler effect, applies to all types of waves and is named after Christian Johann Doppler, an Austrian scientist who predicted in 1842 that the color of a luminous body would change in a similar manner, due to the relative motion of the body and the observer.

Simply stated, the Doppler effect works this way: when a source of sound approaches the listener, the waves in front of the source are crowded together so that the listener receives a larger number of waves in the same time than would have been received from a stationary source. This process raises the pitch that the listener hears. Similarly, when the source moves away from the listener, the waves spread farther apart and the observer receives fewer waves per unit of time, resulting in a lower pitch.

The Doppler effect is of great importance in optics. Since the velocity of light is so large, pronounced effects can be observed only for astronomical or atomic bodies that have velocities which are large compared to ordinary speeds. The effect is seen in the shift in the wavelengths of light emitted by moving astronomical bodies. The shift to longer wavelengths of light emitted from distant galaxies indicates that they are receding and hence supports the concept of an expanding universe. Radiation from hot gases shows a spread of wavelengths (Doppler broadening), because the emitting atoms or molecules move at varying speeds in different directions as measured by the observing instrument.

The Doppler effect has many uses in science and a variety of practical applications as well. Measurements of shifts of radio waves from orbiting satellites, for example, are used in maritime navigation, and the effect is also employed in the radar surveillance of automobile speeds. Medically, the effect is used in such techniques as ultrasonography, or the study of motions in deep-lying body structures, and echocardiography, or the study of heart motions. Such techniques make use of ultrasonic waves, or very high-speed sound waves.

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