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Question about speed of light and water

  1. Aug 6, 2006 #1
    Forgive me if this seems to be a "stupid" question, but it is something that I just have to ask. From what I have read on the web, the speed of light changes when it enters water and other materials. Does it really slow, or does it just "seem" to slow? How does this happen?

    Thanks for you help,
    Dao Tuat
  2. jcsd
  3. Aug 6, 2006 #2
    Again I'm going to post on this just to see if I've got my understanding right. The speed that light propogates at is always c, but if it travels through a medium it can be slowed by absorption and re-emission, essentially the photons themselves always travel at the speed of light, but everything about the speed of light in a medium other than the true vacuum relies on absorption and re-emission, essentially a material will slow the process because at some point many photons are not actually propogating, the net gain is that light appears slowed, but never actually is. There is no average speed of light or mean c in any circumstance in reality, light speed is simply that: c= 299 792 458 m/s.

    You can apparently slow light to a speed that is pedestrian in comparison with c, but it takes materials that absorb almost all of the light as it travels, and so the light is tied into the material it is passing through.

    Of course relativistically we can open up more interesting questions, but essentially even in a relative framework the light isn't travelling any quicker than c, but I digress.
    Last edited: Aug 6, 2006
  4. Aug 6, 2006 #3
    Light always moves at C ~300,000km/s.

    If light were a track star running a race, it would start the race at 300,000km/s and end the race at 300,000km/s. However, when light moves through a medium other then the vaccum of space, say glass, its velocity is slowed. Lets examine this... In glass, light is still a track star running a race, but imagine this time it is running a relay race. Each atom in the glass is a teammate to which the track star must pass the baton, and each teammate must pass the baton to the next teammate in line. This baton passing takes up time, but light always travels at 300,000 km/s for every moment up to the actual baton passing, and for every moment after the actual baton passing, until the next passing of course, where it is slowed down again. Once we add up all the delays of passing the baton between several teammates, at the finish line we measure light as slowing down to about 220,000 km/s, in the specific case of glass (crown glass to be precise). In general, the denser the medium (we examined crown glass in our example), the more 'baton-passings' occur which in aggregate appear to slow down light. Diamonds, for example, are so dense a medium that light is slowed down to below 150,000km/s when inside of it due to all the atoms (teammates) that must pass the baton before the light can emerge.

    In a perfect vacuum, there are no teammates thus no baton passing to slow down light so it merrily goes along its way at a constant ~300,000 km/s.

    Hope this helps.

  5. Aug 6, 2006 #4

    Hans de Vries

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    Science Advisor

    There has been a very interesting experiment lately on this:

    A photon that hits an atom in a very dilute gas has a higher momentum
    as in vacuum (proportional to the refractive index of the gas) Light slows
    down and therefore has a shorter wavelength = higher momentum.

    Gretchen K. Campbell, Aaron E. Leanhardt, Jonchul Mun, Micah Boyd, Erik W. Streed, Wolfgang Ketterle: Photon Recoil Momentum in Dispersive Media.
    Phys. Rev. Lett. 94 170403 (2005)


    Regards, Hans.
    Last edited: Aug 6, 2006
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