Doppler effect

Doppler effect, apparent change in frequency of waves of light or sound due to the motion of an observer relative to the source. If either source or observer is approaching the other, the waves are bunched together, like the folds of a squeezed accordion. The observer encounters more waves in a given period of time than would be the case if both observer and source were stationary, so the observed frequency of the waves increases. Waves of high frequency have a shorter wavelength than waves of low frequency. In the case of light, a shortening of the wavelength makes the object seem bluer than normal, because blue light has the shortest wavelength. In the case of sound, the pitch, as of the whistle of an approaching train, will seem higher. When the source and the observer are moving apart, the waves are stretched out, like the folds of an extended accordion. Because the speed of the whole wave train is constant—the speed of light or the speed of sound—the waves pass less frequently. The distance between waves (the wavelength) seems to increase. If the waves are of light, the object appears redder than normal. If the waves are of sound, the note changes to a lower pitch. Named after its discoverer, the Austrian physicist Christian Johann Doppler (1803–53), the effect has proved of particular use to astronomers in analyzing the light of distant objects in space. The light of these objects is reddened by the Doppler effect, from which it follows that the universe is expanding.