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The Doppler effect is a phenomenon related to the effects of moving objects on an electromagnetic field. In this, an observer in relative motion to a wave experiences a change in the frequency or wavelength of the wave. For example, consider a person standing near a road. A honking vehicle approaches the observer, nears and then recedes from the observer. While the vehicle is approaching, the observer hears the horn as a low tone. However, as the vehicle nears the observer, the sound becomes higher and higher pitched and is the highest at the closest point to the person. But after passing that point, the frequency or the pitch of the sound lowers with distance.
The Doppler effect is also known as Doppler shift.
The Doppler effect is a very common phenomenon occurring in everyday life. It is not just limited to electromagnetic waves. Objects producing waves of any kind can create the phenomena of the Doppler effect. In this phenomenon, a wave source passes from near an observer in relative motion to the object. As it closes in toward the observer, a crest of the wave is generated from a point closer to the observer than the previous one. In this way, the time of arrival to the user is reduced, along with the distance between each wave. This results in an increase of frequency while the object is moving toward the observer. On the other hand, the converse of this process occurs when the object is moving away from the observer. The distance between the waves is increased as the waves spread out and disperse, and the frequency of the wave is reduced.
This effect was named after Christian Doppler of Austria, who proposed this phenomenon in 1842.