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Speed of light question

  1. May 27, 2007 #1
    When moving through empty space, light travels at 3*10^8 meters/second. When moving through something like air or water, the speed of light is slowed down. Is there some type of equation that could tell me the speed of light when it moves through a gas or liquid?

  2. jcsd
  3. May 27, 2007 #2


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  4. May 30, 2007 #3
    nopers, the speed of light remains unchanged through all medium.

    the frequency also remains unchanged through all medium, and therefore its wavelength as well.

    the only thing that changes with the index of refraction is the angle of refraction i.e. the angle at which light leaves the object in relation to the angle at which it entered the object.
  5. May 30, 2007 #4
    I seem to remember and article in National Geographic that said that the speed of light is minutely increasing. The amount was significantly small, although not insignificant.
  6. May 30, 2007 #5


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    You mean the speed of photons remains unchanged. The speed of light (not photon) changes in mediums because photons are absorbed and reemitted by atoms.
  7. May 30, 2007 #6
    That's just wrong; wavelength and speed do change. This is necessary to understand optics, lenses, etc. As for the OP, to calculate and correctly understand "why" the speed changes would seem to require quantum electrodynamics.
  8. Jun 1, 2007 #7
    n = c/v ....... ( minimum post requirement )
  9. Jun 1, 2007 #8
    Speed of light in FREE SPACE doesn't change no matter what FRAME OF REFERENCE.
  10. Jun 2, 2007 #9


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    From Maxwell -

    [tex]c = \frac{1}{\sqrt{\mu\epsilon}}[/tex]
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