A natural phenomenon of electromagnetic waves. It describes the apparent change in frequency or wavelength of a wave that is detected by an observer moving relative to the source of the waves. The frequency of sound and light waves changes when the distance between the source and the receiver changes. The classical example is the sound of a train approaching and then disappearing. The frequency seems to increase as the distance decreases and to decrease as the distance increases.
- "We know from general experience that a ship of moderately deep draught which is steering toward the oncoming waves has to receive, in the same period of time, more waves and with a greater impact than one which is not moving or is even moving along in the direction. If this is valid for the waves of water, then why should it not also be applied with necessary modification to air and ether waves?"
"It is almost to be accepted with certainty that this will in the not too distant future offer astronomers a welcome means to determine the movements and distances of such stars which, because of their unmeasurable distances from us and the consequent smallness of the parallactic angles, until this moment hardly presented the hope of such measurements and determinations".
Doppler's hypothesis for sound waves was tested in Utrecht in 1845 by the Dutch scientist Christoph Hendrik Diederik Buys Ballot (1817-1890). The test was done on the railway between Utrecht and Amsterdam, using a locomotive capable of attaining the, at that time, incredible speed of 40 mph, to pull an open cart with trumpeters at different speeds past trumpeters who had perfect pitch. Ballot observed changes in the apparent pitch of the notes played by the musicians as they approached or receded.
In 1848, the French physicist Armand-Hippolyte Fizeau (1819-1896) discovered the same phenomenon in electromagnetic waves, independently of Doppler and Buys Ballot. Fizeau thus generalized Doppler's work by applying his theory not only to sound, but also to light. In France the effect is sometimes called Doppler-Fizeau effect. Edwin Hubble later used the Doppler effect to show that light from distant stars was shifted to the red part of the spectrum (the famous "red shift"), thereby demonstrating that the universe is expanding.
We thank Bill Johnson for information submitted.
- C. Doppler:
Über das farbige licht der Doppelsterne und eininger anderer Gestirne des Himmels.
Read on May 25, 1842.
Abhandlungen der Königlich Böhmischen Gesellschaft der Wissenschaften, Prague, 5th series, 1842, 2: 465-482. Also published separately in Prague in 1842.