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Why does light travel at varying speeds through different materials?

  1. Aug 17, 2008 #1
    Why does light travel at varying speeds through different materials??

    why does light travel at different speeds through different materials??

    for example, it travels at 38 mph through super cooled sodium.

    also, if light is massless, why is it susceptible to gravity?? i dont know the formulae to examine, but i'd think mass is a factor when calculating gravity, and if so would a zero mass mean gravity has no effect, therefore meaning light does have mass??
     
  2. jcsd
  3. Aug 17, 2008 #2
    Re: Why does light travel at varying speeds through different materials??

    Does light speed vary, or does its path and interactions vary?
     
  4. Aug 17, 2008 #3
    Re: Why does light travel at varying speeds through different materials??

    good counter-question
     
  5. Aug 17, 2008 #4
    Re: Why does light travel at varying speeds through different materials??

    It might help you to visualize a ray of light traveling through a mess of atoms. If the light is hitting atoms, won't it take longer for it to get through the material, even if it's traveling at a constant speed while it isn't hitting anything?
     
  6. Aug 18, 2008 #5
    Re: Why does light travel at varying speeds through different materials??

    Refer to the fourth post in the FAQs thread for a pretty decent explanation.

    https://www.physicsforums.com/showthread.php?t=104715
     
  7. Aug 18, 2008 #6
    Re: Why does light travel at varying speeds through different materials??

    informative link, thanks.

    i'm still puzzled as to why light is susceptible to gravity, yet defined as massless though.
     
  8. Aug 18, 2008 #7
    Re: Why does light travel at varying speeds through different materials??

    1)You are making a non-relativistic consideration, that is a newtonian consideration, while things are more complicated (General Relativity).
    2) But even within the simple newtonian physics, you can take a particle of mass m, look at the trajectory it follows in a grav. field, then reduce its mass to m/2 and look that its trajectory is still bent (if you remember, an object's acceleration in a grav. field does NOT depend on its mass); then make the mass go to zero: nothing changes, so at the limit m = 0, your massless particle still have to follow the grav. field. and so bend its trajectory.
    3) The prediction of General Relativity is that the amount of bending of the particle's trajectory, at the limit m --> 0, is double than that calculated in the previous way.
    4) The famous experiment of star's light deviated from the sun during an eclipse in the 30', showed that the correct prediction was that of Albert Einstein's General Relativity Theory.
     
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