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Speed of Light?

  1. Dec 1, 2003 #1
    Speed of Light???

    Would someone clear up the speed of light??? Is it dependent on the medium in which it travels, or not??? Depending on what you read. It says that light always travels at "c". Or it can be slowed down "refracted" depending on the medium.

  2. jcsd
  3. Dec 1, 2003 #2
    The speed of light is always a constant irrespective of the frame of reference or medium. The apparent slowing down of light due to refraction is due to the fact that the atoms inside the medium absorb the light and then emit the light after some time delay. This time delay in the transmission of light across the medium gives a feeling that light has been slowed down by the medium!!!. So as you see, light never travels wit a speed < or > c. It infact travels at c irrespective of where it is headed for, or who makes the measurement, or the medium in which it is travelling in.


  4. Dec 1, 2003 #3
    so it does not in fact slow down at all? only delayed? then wud tht not be slowing down?
  5. Dec 1, 2003 #4


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    Take your choice. The crucial point of relativity is that the speed of light in vacuum is constant irrespective of the frame of reference.

    From a macro point of view, light is slowed down by passing through matter. From a micro point of view, light travels at c from atom to atom and what we perceive as slowing down is the slight delay when light is absorbed and then retransmitted by atoms.
  6. Dec 1, 2003 #5
    thanks for that.
    but if someone asks me does light slow down in a different medium i say........
  7. Dec 1, 2003 #6
    ... you say that "Yes light is slowed down by the medium as percieved by us at the macro level - However what actually happens is like .....(the previous explanation given that the apparent slowing of light due to delayed transmission of light through the medium) "

  8. Dec 1, 2003 #7
    thanks people
  9. Dec 1, 2003 #8


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    Ask that person what they mean by "light."
  10. Dec 2, 2003 #9


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    And, of course, require that they define "slow down"!
  11. Dec 2, 2003 #10
    The reason refraction made sense to me was that as light entered a denser medium its speed decreased and due to its frequency remaining the same the wave length also decreased, balancing the equation c=f.<lambda>. This change in wavelength was then responsible for the change in direction:


    Why should refraction exist if the delay is due to light is being transmitted from atom to atom? If this were the case I would expect light to emerge from a medium on a random vector.
  12. Dec 2, 2003 #11


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    That sounds like a good question to me. My guess would be that, since momentum is conserved, even when a photon is absorbed, the momentum still exists, so there is some preference to emit a photon with that momentum. That's just a guess, though.

    Now I got to thinking, about the wavelength. Is the wavelength truly shorter in a higher index of refraction? Or do we just use the shorter wavelength as a pseudo wavelength to do calculations? I guess the first question I should ask: can the light in a refractive medium be coherrent? I've seen laser light in air, but I'm talking about significant refraction. Is laser light coherrent in water?
  13. Dec 8, 2003 #12
    Look at it this way: A photon hits a flat piece of glass head on. The photon is absorbed by a particular atom, then after a very brief time, is re-emmited. It is directed to the next atom which does the same thing and so on until it exits the glass.
    The photon emmited and re-emmited always is going the speed of light. But, the CUMULATIVE time delay of all the atoms involved in absorbtion and re-emmission accounts for the apparent, but NOT REAL, slowing down of the photon itself.
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