DOPPLER , CHRISTIAN JOHANN

Christian Doppler, the son of a stonemason from the Austrian city of Salzburg, studied mathematics at the Vienna Polytechnic. In 1835 he started teaching at a school in Prague and six years later was appointed professor of mathematics at the Technical Academy there.

Doppler's fame comes from his discovery in 1842 of the Doppler effect – the fact that the observed frequency of a wave depends on the velocity of the source relative to the observer. The effect can be observed with sound waves. If the source is moving toward the observer, the pitch is higher; if it moves away, the pitch is lower. A common example is the fall in frequency of a train's whistle or a vehicle siren as it passes. Doppler's principle was tested experimentally in 1843 by Christoph Buys Ballot, who used a train to pull trumpeters at different speeds past musicians who had perfect pitch.

Doppler also tried to apply his principle to light waves, with limited success. It was Armand Fizeau in 1848 who suggested that at high relative velocities the apparent color of the source would be changed by the motion: an object moving toward the observer would appear bluer; one moving away would appear redder. The shift in the spectra of celestial objects (the Doppler shift) is used to measure the rate of recession or approach relative to the Earth.