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Wavelength shift for moving objects.

The wavelength of light emitted by a moving object is shifted. This effect is called the doppler shift. We don't normally notice this for light. But it is easy to observe for sound: The reason for this effect is easiest to understand from a picture. The object emits one wave per second. At successive one second times it was located at the positions of the blue dots ...

To get it really right, you should understand the Special Theory of Relativity, but the picture gives the qualitative idea.

Use of the Doppler shift in astronomy

Here is what the spectrum of a star that is moving toward us might look like (somewhat simplified):

Here is what the spectrum of a star that is moving away from us might look like (somewhat simplified):

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu