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Will light be trapped in a room full of mirrors?

  1. Mar 21, 2010 #1
    Will light be trapped in a room full of mirrors? Can this experiment be done?
     
  2. jcsd
  3. Mar 21, 2010 #2
    If the mirror is cool enough, then yes.
     
  4. Mar 21, 2010 #3
    wouldn't radiation pressure make impossible to "trap" light? since every reflection off the mirror would need the mirror to experience twice the force of the intensity of the light wave. which, although small, would cause movement to the mirror. kind of like a solar sail.
     
  5. Mar 21, 2010 #4

    Nabeshin

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    Sure but you could easily imagine compensating for this with some external pressure. The real kicker is that there exist no perfect mirrors, so eventually the photons will not be re-emitted. Also, it is impossible to construct a geometry such that you can emit a photon and have it never impact the photon source. So you have to imagine a situation where you shine a light into a chamber and very shortly afterwards "close" the chamber before the light has a chance to escape.
     
  6. Mar 22, 2010 #5
    I think I read a thread earlier on this forum about trapping light in a PERFECTLY ROUND ball with a PERFECT vacuum inside. And the inside surfaces of this ball are PERFECTLY reflective. Now just reading that you would know that this is impossible.

    I don't remember the answer exactly but I think it was there would be a lot of energy trapped within the ball. I can't remember the end result though ...
     
  7. Mar 22, 2010 #6
  8. Mar 22, 2010 #7

    mgb_phys

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    There are still losses with TIR - energy is released as surface plasmons into the surrounding material, and of course there are absorption losses in the material.

    You can do pretty well though with optical fibre at the right wavelength
     
  9. Mar 23, 2010 #8

    mgb_phys

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    Erm no, you can't see light unless it goes into your eye.
    If you put your head in the beam you would see the light, but that would also stop it.
     
  10. Mar 23, 2010 #9

    sophiecentaur

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    To get the 'delay' I think you'd have to not watch the beam for a while, whilst it's decaying, and then 'stick your head in the way' (or use some fast mirror arrangement) to see what's left of the light energy. Otherwise, the very fact of observing the process would reduce the decay time significantly.
     
  11. Mar 23, 2010 #10
    well if you were to light a bulb in a room with 100% mirrors comprising its inner surface, as soon as you turn the bulb off then I'm sure the light would disappear!

    I wish I knew enough basic physics to know where that light goes, lol.
     
  12. Mar 23, 2010 #11

    sophiecentaur

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    I still wonder what happens in the fridge when I shut the door. The greens stay green so perhaps it stays on. There's not room inside for me to stay in and check, though.
     
  13. Mar 23, 2010 #12
    1. The radiation pressure wouldn't be a problem as long as you attached the two mirrors together. Then the momentum can be conserved as the apparatus goes one way the photon goes the other. The other solution of having a pressure outside wouldn't work because the impulse of the photon would still be transferred to the mirror which would expand into the pressurised gas, doing work...


    2. With a highly reflective mirror and a long beam path I bet you could get an observable effect though. You'd set the source moving perpendicular to the beam and have a detector facing the same way, following and slowly catching up, so it collected reflected light in a time domain which is proportional to the time the light has been reflecting, until it reaches the source and collects light after only a single reflection.


    3. Theoretically I can't see why not if you have an infinite amount of perfectly aligned (but imperfect) one-way mirrors, the problem becomes a 1D random walk for the photon with a bias towards the cavity since it is more likely to be reflected than transmitted. Then in a Markov chain the state for the photon to be in the cavity is recurrent, so the answer is "yes".
     
  14. Mar 23, 2010 #13
    How do you perfectly align imperfect mirrors.
     
  15. Mar 24, 2010 #14
    I meant imperfect = they have less than 100% reflectivity. I didn't make that clear.
     
  16. Mar 24, 2010 #15
    There is no real answer to this question.
     
  17. Mar 24, 2010 #16

    I suppose you are correct, although it would be very hard to measure this.
     
  18. Jul 31, 2012 #17
    This has actually been done but it isn't easy and it isn't for long (in human terms at least):

    http://www.cosmosmagazine.com/news/1101/envisaged-einstein-photon-trap-built [Broken]
     
    Last edited by a moderator: May 6, 2017
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