| 单词 | Doppler effect |
| 释义 | Doppler effect The apparent change in the observed frequency of a wave as a result of relative motion between the source and the observer. For example, the sound made by a low-flying aircraft as it approaches appears to fall in pitch as it passes and flies away. In fact, the frequency of the aircraft engine remains constant but as it is approaching more sound waves per second impinge on the ear and as it recedes fewer sound waves per second impinge on the ear. The apparent frequency, F, is given by F = f(c – uo)/(c – us) , where f is the true frequency, c is the speed of sound, and uo and us are the speeds of the observer and the source, respectively.Although the example of sound is most commonly experienced, the effect was suggested by Christian Johann Doppler (1803–53), an Austrian physicist, as an attempt to explain the coloration of stars. In fact the Doppler effect cannot be observed visually in relation to the stars, although the effect does occur with electromagnetic radiation and the redshift of light from receding stars can be observed spectroscopically. The Doppler effect is also used in radar to distinguish between stationary and moving targets and to provide information regarding the speed of moving targets by measuring the frequency shift between the emitted and reflected radiation. For electromagnetic radiation, the speed of light, c, features in the calculation and as there is no fixed medium to provide a frame of reference, relativity has to be taken into account, so that F = fv[(1 – v/c)/(1 + v/c)] , where v is the speed at which source and observer are moving apart. If v2/c2 is small compared to 1, i.e. if the speed of separation is small compared to the speed of light, this equation simplifies to F = f(1 – v/c) |
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