# Doppler Effect

## Doppler Effect (Encyclopedia of Science)

The Doppler effect is an effect observed in light and sound waves as they move toward or away from an observer. One simple example of the Doppler effect is the sound of an automobile horn. Picture a person standing on a street corner. A car approaches, blowing its horn. As the car continues moving toward the person, the pitch of the horn appears to increase; its sound goes higher and higher. As the car passes the observer, however, the effect is reversed. The pitch of the car horn becomes lower and lower.

Explanation

All waves can be defined by two related properties: their wavelength and frequency. Wavelength is the distance between two adjacent (next to each other) and identical parts of the wave, such as between two wave crests (peaks). Frequency is the number of wave crests that pass a given point per second. For reference, the wavelength of visible light is about 400 to 700 nanometers (billionths of a meter), and its frequency is about 4.3 to 7.5 1014 hertz (cycles per second). The wavelength of sound waves is about 0.017 to 17 meters, and their frequency is about 20 to 20,000 hertz.

The car horn effect described above was first explained around 1842 by Austrian physicist Johann Christian Doppler (1803853). To describe his theory, Doppler used a diagram like the one shown in the accompanying figure of the Doppler effect. As a...

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