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Light In A Medium

  1. Jun 14, 2005 #1
    Second Postulate Of Special Relativity:
    "Speed of light in vacuum remains same for all inertial reference frames"

    The speed of light in glass is 2 x 10^8 m/s . So how is now the above postulate applicable?.....Is it true that if one was sitting inside the glass slab , he would see the light's speed to be 'c'? and not 2 x 10^8 m/s ? ...

    Is the slowing in speed of light apparent?...just because we are sitting outside it but the person inside the glass slab would see the correct light speed>??..

    And suppose person A is inside the glass slab and person B is outside the slab , that is in vacuum ..and there is a light beam which goes across in the vacuum ..now what will be the light speed for person A? ...for B the light's speed will be faster outside the glass slab and slower inside the glass slab , but for person A in the glass slab , what will be the case??

    and how is all of the above in accordance with the second postulate?

    shall i say " Light speed in glass remains same for all inertial frame observers inside glass slab" ..?
  2. jcsd
  3. Jun 14, 2005 #2
    Please dont talk about refractive indices ..I know how to caclulate light speed in different mediums...I just want to connect it all with relativity ...how is second postulate applicable?
  4. Jun 14, 2005 #3


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    You can treat the speed of light in glass just like any other speed except c. It will vary with the observers speed, and it will vary with the medium´s speed.
    All relativistic effects follow from the constancy of the spped of light in vacuum and are not applicable to any other speeds which don´t remain constant.
  5. Jun 14, 2005 #4
    So you mean relativistic effects are not applicable inside a medium but only in vacuum?....I dont think so.
  6. Jun 14, 2005 #5
    The person in the glass slab will also see light in the glass slab travel slower than light in a vacuum. If both the person in the glass and the person outside the glass are at rest relative to each other, they will agree on the speed of both light beams. The person in the glass slab will say the light beam in the vacuum moves at c, while the light beam in the glass slab moves at some velocity < c, and the person in the vacuum will agree with this.
  7. Jun 14, 2005 #6
    Relativity will still be applicable, but someone inside a medium can't use the speed of light in his medium as c; the constant c must be the speed of the propagation of light in vacuo.
    Last edited: Jun 14, 2005
  8. Jun 14, 2005 #7


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    If the person in the glass is small enough, he'll see light traveling at the same speed as it does in a vacuum - but every now and then interacting with a molecule. The net effect of that interaction on the macroscopic scale is an apparent - but not real - slowing of the speed of light.
  9. Jun 14, 2005 #8
    That means if the person is inside the glass slab , he will see the light speed as 'c'?

    I think you mean in the last line 'glass' and not 'vacuo' ..?
  10. Jun 14, 2005 #9
    No, I didn't mean glass. c is the speed of light in a vacuum, regardless of what medium you are in. Light propagates in air slower than in a vacuum, but even though we are in the air, c is still the speed of light in a vacuum, and the speed of light in air will be slightly less than c, as measured from both us and someone in space.
  11. Jun 14, 2005 #10


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    The second postulate is applicable because it specifcally talks about the speed of light in a vacuum. Clearly to be internally consistent the velcoity of light through a medium must transform in the same way as any other velocity, so no the speed of light through a medium is not generally constant. That's all that really need be said as though SR references light in it's postulate it is not a theory of light in itself.
  12. Jun 14, 2005 #11


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    I believe jcsd has correctly given the answer. I just wanted to add that the refractive index of a material (and hence the speed of light in that material) is frequency dependent.

    So if red light and green light go from vaccuum into glass... They have the same speed c in vacuum, but when they enter the glass they'll have different speeds... so speed of light is not constant in a medium.
    Last edited: Jun 14, 2005
  13. Jun 14, 2005 #12


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    No, he'd have to be around the same size as a molecule and be able to watch light go from molecule to molecule at C, then pause as it is absorbed and re-emitted by each molecule.
  14. Jun 14, 2005 #13


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  15. Jun 15, 2005 #14
    So that means that if we are outside the glass slab , we are large enough that we donot account for the photon-level speeds but all we see is the net effect of photon being obstructed in their path by the molecules of the medium , as a result we apparently see that light has slowed down .
  16. Jun 15, 2005 #15
    Whether light travels in a medium at velocity c subject to delays due to absorption and re-emission as a new photon... or whether the photon itself is slowed by fields within a medium is not known. One can construct a theory along the lines of an interaction between the electric field of the photon and the electric field of the electrons in the material. This is tangentially related to the question of whether photons fall like particles in a gravitational field or whether they only change frequency when moving from one gravitational potential to another.
  17. Jun 16, 2005 #16


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    Sorry, but this IS known! Lena Hau and her students just didn't go into their labs blindly and manage to slow down light via trial and error without any clue on what they should manipulate. We also know enough to construct metamaterial to not only make photonic band gap materials, but to make them have negative index of refraction!

    The study of optical conductivity in matter is a very well-known subject in condensed matter physics. It is so well-known, in fact, that we use this to study the properties of materials. Do not associate what you do not know with the state of knowledge in physics.

  18. Jun 16, 2005 #17


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    Yep...and since high schools never go beyond that when teaching it, it leaves many with a false understanding of what is really going on.
  19. Jun 16, 2005 #18
    Same problem with me.I was only taught about snell's law and how light slows down , never knew about the inner mechanism.Thanx for the new knowledge.
  20. Jun 16, 2005 #19

    Meir Achuz

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    SR says all frames with constant relative velocities are equivalent.
    That is not true in a glass plate. SR is irrelevant inside matter.
    Any EM calculations must be done in the rest frame of the glass.
    Then, any questions about what happens if the glass moves, can be answered by LT.
    This is how the Fizeau water tube experiment is explained.
  21. Jun 16, 2005 #20
    If photons are slowed down because they are absorbed and re-emitted won't they come out in all sorts of random directions?
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