Christian Doppler
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Johann Christian Andreas Doppler (November 29, 1803 – March 17, 1853) was an Austrian mathematician and physicist, most famous for the hypothesis of what is now known as the Doppler effect which is the apparent change in frequency and wavelength of a wave that is perceived by an observer moving relative to the source of the waves.
Life
Christian Doppler was born in Salzburg, Austria, as the son of a well-to-do stone-mason, Johann Evangialist Doppler and his wife, Therese. A baptismal certificate indicates that Doppler was christened "Christian Adreas." Doppler was not of strong constitution, and did not follow his father into the construction trades. Instead, he was allowed to follow an academic track. His father asked the advice of Simon Stampfer, a local educator, as too the boy's future. Stampfer recognized Doppler's potential and recommended that his father send him to Vienna.<<<Maulik, Dev. 1997. Doppler ultrasound in obstetrics & gynecology. New York: Springer. 2. ISBN 0387942408.>>> After completing high school he spent three years at the Polytechnic Institute in Vienna, and returned to Saltzburg, where he completed his studies while tutoring mathematics and physics. He returned to the Polytechnic Institute for four years, where he taught higher mathematics. It was during this period that he wrote his first papers in mathematics and electricity.
Relative speed and the frequency of light
Unable to find a teaching position, he contemplated a move to the United States. But after making preparations for the trip, he unexpectedly secured a post in 1835 as professor of mathematics and accounting at the State Secondary School in Prague. In 1841, he was employed at the State Technical Academy as a mathematics professor. A year later, in 1842, he read his first work on the changes in frequency of waves through a medium measured by an observer moving with respect to the waves' source. This has come to be known as the Doppler effect.
Waves are disturbances in a medium such as air or the surface of water, that move through the medium, even though the actual parts of the medium experience only a small movement. Sound moves through air by wave motion, the disturbances in a pond ripple across the water's surface as a wave motion, and one theory of light holds that light is transmitted by wave motion through a medium, with each color representing a succession of waves with a uniform frequency, the number of times a wave passes a particular point in a second. Sound waves are in the thousands per second, but light waves are much closer together and move much faster, producing frequencies in the billions per second.
Doppler's paper, presented before the Royal Bohemian Society of Sciences on May 25, 1842, was entitled On the colored light of the double stars and certain other stars of the heavens.<<<Hennerici, M., and Stephen P. Meairs. 2001. Cerebrovascular ultrasound theory, practice, and future developments. Cambridge: Cambridge University Press. 88.>>> His work was an extension of that of James Bradley, the eighteenth century astronomer who measured the speed of light by noting the abberation that affects the apparent positions of all stars due to the orbital motion of the earth. Bradley made his calculations assuming a particle theory of light. But Doppler applied similar reasoning to show that the frequency of light would change if the observer was not stationary relative to the light source.
Doppler's work was later published under the title: On the colored light of the binary refracted stars and other celestial bodies - Attempt of a more general theory including Bradley's theorem as an integral part. In Doppler's own words, "...nothing seems to be more intelligible than that to an observer the path length and the interim durations between two consecutive breakings of a wave must become shorter if the observer is hurrying toward the oncoming wave, and longer if he is fleeing from it..." In other words, if one were swimming toward waves coming into a beach, one would hit them more frequently, whereas if one were swimming in the direction they moved toward the shore, one would encounter them less often.
As the frequency is dependent on the number of beats per unit time, and, in the case of light, is related to the color perceived, Doppler concluded that the red color of some stars demonstrates that the perceived wavelength is greater as a result of the star and the earth moving away from each other."<<<Schuster, Peter. 2005. Moving the stars Christian Doppler, his life, his works and principle, and the world after. Pöllauberg, Austria: Living Edition. 63.>>>
In this work, Doppler tried to explain the color of stars by first assuming that the natural color of stars is white, and by conjecturing that if the earth and the stars are rapidly approaching each other, their color should be shifted to the shorter, or violet, end of the spectrum, while if receding, to the red, or longer wave-length, end.
Two investigators, C.H.D. Buys-Ballot in 1845 and John Scott Russell in 1848, verified Doppler's prediction with respect to sound. But the former also criticized Doppler's assumptions about the change in color, emphasizing that the entire range of colors in the spectrum would change, but in combination still producing white light. Doppler responded to these objections with several papers of his own. Still, in 1847, the Italian astronomer Benedict Sestini published a catalogue of star colors, and addressed some of Doppler's ideas. Doppler's contention about starlight was also shown to be incorrect on another ground, however. A color change, if it were to occur, would be so slight, due to the great velocity of light as compared with any possible stellar motions, as to be undetectable by the eye.
Some of these points were made in a presentation by H. Fizeau in 1848, who applied Doppler's theory to the total shift in the position of so-called Fraunhofer lines that appear in spectra and that mark specific frequencies of light.
Doppler, however, went on to elaborate on his theory and published numerous papers on a variety of subjects throughout his career, particularly in optics. In 1843, he was made a full professor, and in 1846 he published equations applying to the case where both the observer and the wave source were in motion. Doppler was unsure regarding the application of his principle to transverse waves, where the undulatory motion is perpendicular to the direction of wave propagation. This case was treated by the mathematical physicist B. Bolzano, who announced his findings shortly after Doppler's paper appeared. Bolzano, who was a colleague of Doppler in Prague, predicted that Doppler's method would enjoy great utility in determining the velocity and other characteristics of stellar objects. <<<Merz, John Theodore. 1912. A history of European thought of the nineteenth century. Vol. 3. Edinburgh: Blackwood. 10.>>>
It is this application that was to mark the future application of Doppler's principle more than a shift in the actual color of stars. <<<Campbell, William Wallace. 1913. Stellar motions, with special reference to motions determined by means of the spectrograph. New Haven: Yale university press. 12-15.>>>
Also in 1846, Doppler calculated the resolution that could be obtained with a photographic plate, estimating that it was 40,000 times more suseptible to impressions than the human eye. He thus recommended that astronomers concentrate on magnifying the image made on a photographic plate instead of trying to increase the power of telescopes. <<<Anonymous. 1847. On the combination of the daguerrotype with the telescope. Civil Engineering and Architect's Journal. 10:102. Wiley and Putnam: New York.>>>
His research career in Prague was interrupted by the revolutionary incidents of March 1848, when he fled to Vienna. There he was appointed head of the Institute for Experimental Physics at the University of Vienna in 1850. During his presence at the University of Vienna, Doppler, along with Franz Unger, played an influential role in the development of young Gregor Mendel who later became the founding father of genetics. (Note: Gregor Mendel studied in the University of Vienna from 1851 to 1853. At that time, Doppler served as a professor there.)
He died from a pulmonary disease in Venice at age 49 on March 17, 1853. His tomb can be found just inside the entrance of the Venetian island cemetery of San Michele.
Legacy
Some scientists have argued that Doppler's principle was merely an extension of that of Ole Roemer, who by measuring the fluctuations in the apparent periods of revolution of Jupiter's moon's in the 1600s, showed that light had a finite velocity. Doppler did take an additional step, however, in applying the wave theory of light to the effect that a variation in velocity would have on a light-source's color. While this was not strictly true, the general idea held that the apparent light frequency would change, a fact more clearly demonstrated by H. Fizeau some years later.
As the history of science often demonstrates, it is a small step that sometimes has an inordinate impact on scientific progress. Doppler's name is practically a household word, with weather reports relying on "doppler radar," while his name remains connected with a wide range of phenomena relating to both sound and light waves.
The power of the Doppler principle, defined more clearly by Fizeau, was taken a step further by British astronomer William Higgins, who in 1868 applied the Doppler effect to obtain concrete measurements of velocity from the shift in spectral lines.<<<Cajori, Florian. 1899. A history of physics in its elementary branches including the evolution of physical laboratoires. New York: Macmillan. 168.>>> Since that time, the impact of Doppler's work on astronomy has been enormous.
See also
ReferencesISBN links support NWE through referral fees
<<Need at least 3 reliable references here, properly formatted.>>
- Schuster, Peter. 2005. Moving the stars Christian Doppler, his life, his works and principle, and the world after. Pöllauberg, Austria: Living Edition.; Webpage of the author)
- Alec Eden, "The search for Christian Doppler," Springer-Verlag 1992. (An English translation of Doppler's 1842 work on the Doppler effect can be found in this book. In this book, Alec Eden also made a detailed study regarding the full name of Christian Doppler. He believed that his full name should be "Christian Andreas Doppler".)
External links
- Christian Doppler and his birthplace in Salzburg
- John J. O'Connor and Edmund F. Robertson. Christian Doppler at the MacTutor archive
- Born between Salzburg and Braunau am Inn
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