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# Doppler’s Effect

Updated on July 2, 2015

## Doppler’s Effect

The Doppler’s effect is the apparent change in frequency of sound, light or radio waves caused b the relative motion of the source, medium or observer. For example, when an approaching train blows its whistle, the pitch of the whistle seems higher as the train comes towards you. The pitch seems to lower when the train passes and goes away from you. The cause of this effect was first studied in 1842 by Christian Doppler, an Austrian physicist and today known as Doppler’s effect.

According to Doppler’s effect whenever sound source moves towards or away from the observer, the pitch of the sound as heard by the observer is slightly different than the one emitted by the source. When the source of sound is approaching each wave sent out by the source has a shorter distance to travel than the wave that was sent out earlier. Each wave reaches the listener a little sooner than it would have, if the source had not been moving. The waves seem to be more closely spaced. They have a higher frequency. As the source moves away each wave starts a little farther away. Each wave seems to be longer than it would ordinarily be. The pitch is lowered. When the source and observer are at rest or in motion in the same direction such that the speed of both is the same, the pitch of the sound does not change.

Doppler’s effect is true in case of light waves also but since the velocity of light is very large as compared to sound, it can not be detected by simple methods. Astronomers study the motion of stars and galaxies by measuring the apparent change in frequencies of light due to motion. It has been observed by the scientists that galaxies are moving away from us. This has been concluded on the basis of red shift and on the basis of the violet shift, we can find out if any star is coming towards us in this vast galaxy of stars.

The rotation of the sun can be measured with the help of Doppler’s effect. For this purpose the wavelength of light coming from the tow edges of the sun is measured.

The speed of fighter planes and submarines is measured by using Doppler’s effect in the battlefield and the Doppler’s effect also helps to determine the position of the fighter planes and submarines, particularly, during a war. An instrument called the laser Doppler velocity meter has been developed with the help of laser beams which is used basically to determine the velocity of fast moving objects.

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