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People Behind the Science

Johann Christian Doppler (1803–1853)

ohann Doppler was an Austrian physicist Doppler explained the effect that bears
Jwho discovered the Doppler effect, which his name by pointing out that sound waves
relates the observed frequency of a wave from a source moving toward an observer
to the relative motion of the source and will reach the observer at a greater frequency
the observer. The Doppler effect is readily than if the source is stationary, thus increas-
observed in moving sound sources, produc- ing the observed frequency and raising the
ing a fall in pitch as the source passes the pitch of the sound. Similarly, sound waves
observer, but it is of most use in astronomy, from a source moving away from the observer
where it is used to estimate the velocities reach the observer more slowly, resulting in a
and distances of distant bodies. decreased frequency and a lowering of pitch.
Doppler was born in Salzburg, Austria, In 1842, Doppler put forward this explana-
on November 29, 1803, the son of a stone- tion and derived the observed frequency
mason. He showed early promise in math- mathematically in Doppler’s principle.
ematics and attended the Polytechnic The first experimental test of Doppler’s
Johann Christian Doppler
Institute in Vienna from 1822 to 1825. principle was made in 1845 at Utrecht in
Despairing of ever obtaining an academic Holland. A locomotive was used to carry a 1848 that shifts in the spectral lines of stars
post, he decided to emigrate to the United group of trumpeters in an open carriage to could be observed and ascribed to the Dop-
States. Then, on the point of departure, he and fro past some musicians able to sense pler effect and so enable their motion to
was offered a professorship of mathemat- the pitch of the notes being played. The be determined. This idea was first applied
ics at the State Secondary School in Prague variation of pitch produced by the motion of in 1868 by William Huggins (1824–1910),
and changed his mind. He subsequently the trumpeters verified Doppler’s equations. who found that S irius is moving away from
obtained professorships in mathematics Doppler correctly suggested his prin- the solar system by detecting a small red-
at the State Technical Academy in Prague ciple would apply to any wave motion shift in its spectrum. With the linking of the
in 1841 and at the Mining Academy in and cited light and sound as examples. He velocity of a galaxy to its distance by Edwin
Schemnitz in 1847. Doppler returned to believed all stars emit white light and that Hubble (1889–1953) in 1929, it became
Vienna the following year and, in 1850, differences in color are observed on Earth possible to use the redshift to determine
became director of the new Physical Insti- because the motion of stars affects the the distances of galaxies. Thus, the prin-
tute and Professor of Experimental Phys- observed frequency of light and hence its ciple that Doppler discovered to explain
ics at the Royal Imperial University of color. This idea was not universally true, an everyday and inconsequential effect in
Vienna. He died from a lung disease in as stars vary in their basic color. However, sound turned out to be of truly cosmological
Venice on March 17, 1853. Armand Fizeau (1819–1896) pointed out in importance.

Source: From the Hutchinson Dictionary of Scientific Biography. © Research Machines plc 2003. All Rights Reserved. Helicon Publishing is a division of Research Machines.

Shock wave front

FIGURE 5.25 A sound source moves with velocity greater than the speed of sound in the medium. The envelope of spherical wave front
forms the conical shock wave.

132 CHAPTER 5 Wave Motions and Sound 5-18