JOSE ANTONIO PENAS / SCIENCE PHOTO LIBRARY JOSE ANTONIO PENAS / SCIENCE PHOTO LIBRARY
Doppler effect experiment. Illustration of the 1845 experiment carried out to prove the existence of the Doppler effect. This effect, where the observed frequency of a wave depends on the relative speed of the source and the observer, was first proposed in 1842 by Austrian mathematician and physicist Christian Doppler (1803-1853). Doppler had proposed this effect in relation to the stars (where it would one day be applied), but the first demonstration was carried out in 1845 in relation to sound waves. Dutch meteorologist Christophe Buys Ballot (1817-1890) arranged for musicians to play their instruments on board a train on the Utrecht-Amsterdam line. When heard from a station platform, the music was higher in pitch as the train approached (blue) and lower in pitch as it receded (red).
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