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Doppler effect in light

  1. Nov 18, 2008 #1
    I do not fully understand the Doppler effect in light.
    A theoretical question:

    If I was traveling near the speed of light, towards a source of light. Would all the wavelengths in the visible spectrum be shortened and therefor, everything shifting towards red?

    I thought the speed of light was supposedly constant, no matter the relative velocity of the observer.
  2. jcsd
  3. Nov 18, 2008 #2

    Doc Al

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    The observed wavelengths will be shorter, which means a shift towards blue.

    The speed is constant. It's the observed wavelength that changes, not the speed.
  4. Nov 18, 2008 #3
    Oh yeah, red is longer wavelength...

    What I don't understand is how the speed of the wave propagation can remain constant relative to the observer. It seems that if you are moving towards the source, your relative velocity would be higher.
  5. Nov 18, 2008 #4

    Doc Al

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    That the speed of light is independent of the speed of the source or observer is counterintuitive. It requires special relativity to make sense of it all.
  6. Nov 18, 2008 #5
    Oh I see, so that property is an assumption.
  7. Nov 18, 2008 #6

    Vanadium 50

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    No, it's an observation.
  8. Nov 18, 2008 #7
    Einstein seemed to think differently.
  9. Nov 18, 2008 #8


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    Einstein assumed the constancy of the speed of light in deriving the special theory of relativity. Since then it has been tested (observed) experimentally in various ways. The link below refers to many of them.

    Experimental Basis of Special Relativity
  10. Nov 18, 2008 #9


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    Rather than playing word games and "quote the physicist", I think your time would be better spent understanding the physics, rather than the connotation of the word. There are already many sources (and many threads on here) on why the simple Galilean addition of velocities will not work at relativistic speeds.

  11. Dec 1, 2008 #10
    Found a video on youtube with a visual explanation of special relativity. Seemed to clear some things up for me with my original question. Here it is if anyone is interested.

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