Doppler Effect
The Doppler effect is known since the middle of the 19th century. If the source of a sound is in relative motion to the observer, the frequency of the sound is perceived by the observer, either higher or lower, depending on the direction of the relative motion. Since light is also regarded as a wave, similar phenomena appear: either as a blueshift or a redshift. Here we will illustrate a redshift schematically, relying on a classical approximation. The redshift appears at some crucial scenes, in relativity theories and relativistic cosmology (gravity and redshift, the Big-Bang and redshift, etc).
In the following figure, the red spot is the source of light, and the blue spot is the observer. Top: the two are at rest relative to each other; Bottom: the source is receding from the observer at the speed of a half of that of light. Consider what happens during 5 seconds (according to a classical approximation, disregarding the relativistic effect).
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Last modified September 4, 2001.(c) Soshichi Uchii suchii@bun.kyoto-u.ac.jp
