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Science Forum

  1. Standard member JRoma
    Wannabe Theoretician
    09 Aug '13 03:46
    I had a weird thought about light and I was hoping somebody could explain this for me.

    The following thought experiment - suppose there is built a room which is light proof. So this room is dark inside and I light a flashlight in the room. Now the room fills with light. I switch off the flashlight and the room will be dark again. Where did the light go? Why does the light not just bounce around in the room since the room was light proof?

    The light couldn't have seeped out since the light is not seeping in from the outside.
  2. Standard member DeepThought
    Losing the Thread
    09 Aug '13 04:02
    It depends on the properties of the walls in the room. If they are absorbing then the room will dim. If they are perfect mirrors then the room will remain illuminated. If the walls are thermally isolated including radiatively from the outside they will re-emit any radiation they absorb back into the room as black-body radiation, what the spectrum is depends on the walls temperature. It is a matter of what is absorbing and what is emitting.
  3. Standard member JRoma
    Wannabe Theoretician
    09 Aug '13 04:06
    Thanks, that makes sense.
  4. Standard member DeepThought
    Losing the Thread
    09 Aug '13 04:16
    Originally posted by JRoma
    Thanks, that makes sense.
    No problem, bear in mind that for room temperature walls the re-emitted radiation will be in the infra-red not the visible. I looked at your profile, I agree about the Najdorf and don't know about the Grunfeld, but nice Nymph!
  5. 09 Aug '13 07:10
    Originally posted by DeepThought
    If they are perfect mirrors then the room will remain illuminated.
    It is very difficult to make perfect mirrors. The best mirrors rely on total internal reflection. So maybe the best experiment would be an optic fibre ring. There is of course some loss even with optic fibres, so the question is, how long would light last in a ring of optic fibre. My guess is not very long at all. I believe that an optic fibre from Cape Town to Europe carries the signal in about 0.3 of a second, and some of that is the devices on the way. And I believe there is some loss over that distance.
    So my guess would be that light in an optic fibre ring, or very long optic fibre would all be lost in about a minute.

    I believe lasers also use mirrors with light bouncing back and forth.
  6. 09 Aug '13 07:42
    First of all, to make such a "dark room", you need something at T = 0 to suppress the black-body radiation. This is pretty hard, but let's say it's at least theoretically possible. You then toss in your torch (also at T = 0), which emits some light. Now if the walls are perfect absorbers, they will absorb the light, heat up, and then start emitting black-body radiation. If you have some heat sinks attached to the walls to keep them at T = 0, over time the room will dim again.
  7. 09 Aug '13 08:22
    The post that was quoted here has been removed
    Perhaps if you hadn't dropped out of community college, you could've recognized some "real science".
  8. 09 Aug '13 08:37 / 1 edit
    The post that was quoted here has been removed
    What the hell are you doing in this thread? -as if you have any valid opinion about the physics of light and black-body radiation
  9. Standard member RJHinds
    The Near Genius
    09 Aug '13 15:02 / 1 edit
    Originally posted by KazetNagorra
    Perhaps if you hadn't dropped out of community college, you could've recognized some "real science".
    I graduated with an Associate Degree. And I might add, I got more credits than required for I took as many courses as was possible during that time, because the tuition was not any more than if I had taken the minimum number of courses.

    The Instructor
  10. 09 Aug '13 16:08
    Originally posted by RJHinds
    I graduated with an Associate Degree. And I might add, I got more credits than required for I took as many courses as was possible during that time, because the tuition was not any more than if I had taken the minimum number of courses.

    The Instructor
    That would be more convincing if you didn't regularly fail to comprehend junior school maths.

    If you ever learnt anything you have long since forgotten it.
  11. Standard member sonhouse
    Fast and Curious
    09 Aug '13 18:28
    The post that was quoted here has been removed
    So now you are an expert on blackbody radiation? What, are you a blackbody radiation denier?