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Light not in a vacuum

  1. May 9, 2006 #1
    I know light travels at a constant speed when it's in a vacuum, but what about when it isn't? That is to say, would two electromagnetic waves with different frequencies travel through the same medium at the same speed?
  2. jcsd
  3. May 10, 2006 #2


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    The simple answer is no, they would not travel at the same speed - which, btw, is why we see rainbows!

    It's a little more complicated than that but, basically, light waves passing through a medium interact with electronic charges in that medium. The incident electromagnetic field accelerates those charges and they re-radiate electromagnetic energy. In effect, the propagating wave causes new waves to be created as it is absorbed. In turn, the same process applies to the radiated fields.

    The overall radiation field depends on the frequency of the incident wave and so the rate at which the full radiation field propagates through the medium also depends on it.
    Last edited: May 11, 2006
  4. May 10, 2006 #3

    Claude Bile

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    The refractive index any material varies with frequency (this variation is termed material dispersion). It is this variation that allows glass prisms for example to break white light into a myriad of colours (hence Tide's refrence to rainbows).

  5. May 10, 2006 #4
    Ok, thanks. That helps a lot.
  6. May 12, 2006 #5
    There was an interesting article in my New Scientist magazine this week about the speed of light changing over time, and I picked up something about the speed of light being slightly different for different frequencies in some situations. Sorry, I can't back this up online, I'll dig it out properly and post something over the weekend.



    Last edited: May 12, 2006
  7. May 14, 2006 #6
    About speed of electromagnetic waves in material media, there is the Cauchy's Law :

    n = A + B/(L^2)

    where n is the refractive index : n=c/v
    and L the wavelength in vacuum.
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