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Doppler Effect on Light

  1. May 26, 2009 #1
    Hi, I'm new to these forums. I just had a question about the Doppler effect on light (red and blue shifts of light). I've read how it is supposed to work, but I also read in a book that the speed of light is constant in all moving frames.

    So if you've got a galaxy moving towards you, and it's throwing out photons towards you, all of which travelling at c, shouldn't there be no doppler effect because relative to one another, they're all traveling 186,000 miles a second, and would all reach you at the same time anyways? The analogy in the book (I think it was Hyperspace by Michio Kaku) said something about cars travelling at 99% the speed of light, and a train traveling at the speed of light, but to the car it would seem the train is still speeding ahead at 100% the speed of light, and not 1% the speed of light...

    OR is it simply that if they're both going the speed of light, the effect can apply?

    Bear with me, I am probably missing something obvious or just plain not understanding :biggrin:

    Thanks for any replies. (And I hope this was the right category to put this in!)
  2. jcsd
  3. May 27, 2009 #2
    From what I understand.

    If you're in a vehicle traveling very close to the speed of light, time inside the vehicle will go normally, but outside the vehicle time will be slowed down, from an observer inside the vehicle.

    I would love for someone that knows a bit more about this to give a proper explanation, I'm very confused as well hehe.
  4. May 27, 2009 #3
    Hi there,

    The Doppler effect as nothing to do with the speed of light contanst. It simply says that if you are moving toward a light source, the frequency of the light will increase.

    Whether the light is blue, red or purple, it all travels at the speed of light. The only difference is the distance between two peaks. As a matter of fact, the only difference between radio wave, micro wave, visible light, X-rays/gamma-rays lies in this distance between peaks.

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