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Doppler, Christian Johann

frequency speed effect source

(1803–53) Austrian physicist: discovered the Doppler effect.

Doppler was educated at the Vienna Polytechnic and, despite his ability, for some time could only gain rather junior posts in tutoring or schoolteaching. At 32 he decided to emigrate to America, but on the point of departure was offered a senior teaching post in a school in Prague. After 6 years he became professor of mathematics at the State Technical Academy there, and in 1850 professor of experimental physics at Vienna.

His claim to fame rests on a single important discovery, the Doppler effect (1842). This proposed that the frequency of waves from a source moving towards an observer will be increased above that from a stationary source; and waves from a source moving away from an observer will be decreased in frequency. In 1845 a test was made at Utrecht in which an open railway carriage carrying a group of trumpeters was taken at speed past a group of musicians with perfect pitch. It was one of the extraordinary occasions that made the 19th-c approach to physics entertaining and, while unsubtle, it demonstrated the correctness of Doppler’s idea.

Doppler recognized that the effect applies not only to sound but also to light, and (1848) pointed out that the spectral lines of stars should be shifted towards the red end of the spectrum according to the speed at which they are receding from us (the Doppler shift).Observed this for the star Sirius (1868) and later used the redshift to infer the speed of recession of other galaxies from us: the ‘expanding universe’ of cosmology.

The Doppler effect has also been used to measure the speed of the Sun’s rotation and Saturn’s rings, and the rotation of double stars. It forms the basis of police radar speed traps for vehicles; and ‘Doppler satellites’, emitting a fixed radio frequency and whose position is known, are used by ships and aircraft to locate their position and by mapmakers and surveyors to give precise locations using the global positioning system (GPS). Doppler reflection is also used in echocardiography. The frequency of a beam of transmitted ultrasound is compared with the frequency of the beam reflected from the moving blood cells in the blood vessel under examination. This allows the velocity of the blood flow (around 1 m s –1 ) to be measured and is of great value in locating valve and other heart defects, especially in children.

Doqui, Robert (1934–) [next] [back] Donner Pass: The Road to Survival

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