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How can doppler effect occur when c is indepedent of motion?

  1. Dec 16, 2012 #1
    I thought the speed of light was constant independent of the motions of the observer and light source. So I don't understand how the doppler (blue-red) shift can be observed on incomming or receding astronomical light sources?

    Speed also varies depending on the medium it travels through, so what is meant when they say the speed of light is constant?
  2. jcsd
  3. Dec 16, 2012 #2


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    I have often wondered about your first question, myself. I have got what I considered good answers in the past but can no longer remember! I'm sure someone else will answer it here.

    I can answer your second question- the proper statement is that the speed of light in vacuum is constant. We can also argue that the speed of light between atoms is constant, it is the time taken when light is absorbed and then retransmitted that reduces the observed speed of light through a different medium. The remarkable thing about light is that its speed is constant to an observer in any frame of reference. Classical theory says that the speed of light seen by an observer moving toward the light source at, say, 10000 mph, would be 20000 that seen by an observer moving away from the light source at 10000 mph. Relativity says that the two observers will see that same speed of light.
  4. Dec 16, 2012 #3


    Staff: Mentor

    The Doppler effect has to do with the frequency/wavelength of observed light, not its speed. The two are independent, at least as long as we assume the light is moving through vacuum (or some non-dispersive medium).
  5. Dec 16, 2012 #4


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    Consider the following animations. The first shows a light source not moving with respect to two observers. The light waves radiate out at a constant speed in all directions.


    Now what happens when the light source moves with respect to the observers.


    Note how each wave still radiates out at a constant speed after it leaves the source. But each successive wave is emitted when the source is closer to the blue observer and further from the red one than it was for the previous wave emission. Thus the spacing between the waves is closer for the blue observer than it is for the red observer and you get blue and red shift.
  6. Dec 16, 2012 #5
    Thanks for the answers!

    I was going to ask why the frequency would change but the animation answered that very well, so thanks for posting those Janus. It's interesting the doppler effect happens with sound for entirely different reasons.
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