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Refraction and c

  1. Mar 23, 2015 #1
    I've recently been stumped after hearing about refraction and its effects on light. I have little knowledge of physics and after doing my own research I thought I could best get an answer here. Any help would be appreciated, or even corrections on what I thought I knew.

    Essentially, I know that the speed of light is about 299,792,458 m/s in a vacuum. However, I've been told that when light travels through a medium that has mass, it moves slower. My previous knowledge would have suggested that light travels at a constant speed (c), but given what I've heard about refraction this does not seem to be true. So for starters, is this correct?

    The theory that I have led myself to believe is that light itself does travel at c, but there is somewhat of an illusion that it does not. What I had thought was that for light to travel from point a to b, should the medium have mass, it would take a longer amount of time than if the medium was a vacuum. Light would however travel at c, but would simply take a longer route by bouncing off of the mass on its way through (or something along the lines of that).

    I suppose that my question could be narrowed down to, does light/a photon always travel at c, regardless of whether or not it is in a vacuum. If clarifcation of what I've said is required then feel free to let me know as I truly am confused on the topic. Thanks.
  2. jcsd
  3. Mar 23, 2015 #2


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    There is an FAQ on the subject: https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/ [Broken]
    Last edited by a moderator: May 7, 2017
  4. Mar 23, 2015 #3


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    It is true that light travels at constant speed in vacuum. It travels at a slower speed in a medium.

    I'm not sure I understand what you mean by "bouncing off the mass on its way through". It is true that most of the interior of any material is vacuum and you can think of the slower speed in the material as time taken up being absorbed and expelled by individual atoms in the material

    The answer to that "narrow" question is NO. Light travels at c in a vacuum, slightly slower in a material medium.
  5. Mar 23, 2015 #4


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    Please note that when we talk about the "speed of light", we usually are referring to the group velocity of light! You send a pulse of light out, and then you measure the transit time of that pulse.

    This is the "speed of light" that is affected by a medium. This is made plainly clear and had to be emphasized unambiguously due to the result of the NEC experiment many years ago with an activated Cs gas that produced an anomalous dispersion. It resulted in a pulse (group velocity) that traveled faster than c, but no part of the wave was moving faster than c.

    This is why a very clear definition and understanding of the quantity that we measure is very important, because at some point, as we push our understanding even more, we will come up with a situation where how we define what we mean becomes significant.

  6. Mar 23, 2015 #5
    There are two ways of looking at this, which can be confusing. One way is to consider the medium that light travels through as polarizable matter having a field of its own. The wave equation (equivalent to Maxwells for the Lorenz gauge) describes light still moving with phase speed c, but the presence charges and currents associated with the medium results in a total field moving at slower phase speed. The much more common alternative is to move the current associated with the polarized medium from the right hand side of the equation to the left hand side, resulting in an effective medium theory. In the effective medium effective light exists that moves with a phase speed c/n, where n is the real part of the index of refraction. This is the usual way of presenting things. So 'effective light' can move at a substantially lower speed than c and this effective c' is also frequency dependent. In ordinary glass optical frequencies move at 2/3 c. It is claimed that in a Bose-Einstein condensate light can even be brought to a complete halt.
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