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Utterly Noob ? What happens to light?

  1. Oct 18, 2007 #1
    Utterly Noob "?" What happens to light?

    Hey, this is my first post, and I know this is a rather rudimentary question. But, I've always wondered, why can't you see light after you turn off a light switch. Is light photons, and if thats the case what happens to them when electricity stops going to the light. They don't just "die" do they? What form (of matter/energy) do they change into after we can't see them anymore?
     
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  3. Oct 18, 2007 #2

    russ_watters

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    Photons travel from the light out to everything in the room (including you) at the speed of light. When they hit objects, they are absorbed or reflected. When they are reflected toward your eyes, you see the object they were reflected from.

    When you turn off the light, no more photons are created by the light and the photons bouncing around the room are quickly absorbed by the walls, furniture, floor, and you.

    Welcome to PF!
     
  4. Oct 18, 2007 #3
    I see, that makes sense. Thank you.
     
  5. Oct 18, 2007 #4
    And other photons escape through the window or out your door... etc...
     
  6. Oct 18, 2007 #5

    DaveC426913

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    "What form (of matter/energy) do they change into after we can't see them anymore?"

    A rise in temperature of the objects in the room.
     
  7. Oct 19, 2007 #6
    Lets assume that I have some kind of material that does not absorb light what so ever. I mean, the material keeps reflecting the light back into the environment.

    Now I use it as the lining in the walls of a certain room that has a small light bulb. Is it possible that the room will stay lit even after I switch off the light bulb?
     
  8. Oct 19, 2007 #7
    You'd have to coat the bulb and its assembly too, or it would absorb the photons as well. Actually, jump to this thread.
     
  9. Oct 19, 2007 #8

    Mk

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  10. Oct 19, 2007 #9

    russ_watters

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    Sure, but you'd never know it since if you entered the room, you'd quickly absorb all the light in it yourself!
     
  11. Oct 19, 2007 #10
    nesna - Another interesting thing with that type of setup. Let's say nothing in the room absorbs light (including yourself). What would you see?

    This is interesting because in order to see a wall, for example, light hits the electrons which are part of the wall and the electrons absorb the photons and emit them. They are emitted such that when that photon reaches your eye you recognize it as, brick or wood, or whatever the wall is made of. But in your example, they are not absorbing and emitting with 100% re-emission, instead the electrons are not absorbing and re-emitting, so the photon carries the same properties as it had when it was generated by the light bulb. So at best you would see (read below before replying to this) a bright blur most likely.

    That being said, you wouldn't actually see anything if your eyes did not absorb the photons, because that's how the electrical impulses reach your brain to tell you what you're seeing. And if you let everything perfectly reflect except for your eyes, then when you turned the lights out you would probably see that bright blur start to dim pretty quickly until it was gone; but it wouldn't be instantaneous.
     
  12. Oct 19, 2007 #11

    DaveC426913

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    Consider the implications of this. That lightbulb has been pouring photons into the room for an arbitrary length of time none of which are being re-absorbed. If you stepped into that room, you'd be stepping into a blast furnace.
     
  13. Oct 20, 2007 #12

    russ_watters

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    You can handle a blast furnace for a nanosecond.
     
  14. Oct 20, 2007 #13

    DaveC426913

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    I dunno. I'd be interested in someone smarter than me putting some numbers to this.

    What is the total energy that a light bulb puts out in, say, 5 minutes?
    Then, how hot would you get if you received that input of energy?
     
  15. Oct 20, 2007 #14

    russ_watters

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    Well, I know its a weird unit transition, but its the units I deal with at work.....

    100 watts for 5 minutes is 28 btuh, which is enough energy to heat 1lb of water 28 degrees F or 28lb of water 1 degree.
     
  16. Oct 20, 2007 #15

    DaveC426913

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    OK. So that would heat your (or at least, my) whole body about 0.15 degrees. That tells us about the temperature rise of a volume, after enough time to have the heat diffuse, but doesn't tell us a lot about the heat as applied to the body's surface area instantly.
     
  17. Oct 20, 2007 #16

    DaveC426913

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    It would be very weird to step into this room.

    It would be very tricky to get into this room without "letting all the light out"**...

    **Can't beliieve I'm saying that with a straight face but, with the premises we've made, this is an apt way of describing the sitch.

    ...but it could be done using an antechamber.

    Also, you'd be wearing a suit lined with the same materials as the walls. And you couldn't see anything either....

    ...

    No wait. It wouldn't be weird at all in this room. There is no way at all for you to view what is happening in this room. The moment you could see, the light would "escape".
     
  18. Oct 20, 2007 #17

    russ_watters

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    Well, the point about your skin was to show that you could heat up you skin 2.8 degrees. Or the outer 1/10th of your skin by 28 degrees. This isn't enough to cause a problem.
     
  19. Oct 20, 2007 #18

    DaveC426913

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    Huh. So if you stepped into that room wearing the outift, and then whipped off the helmet, it might look really bright and then fade fast. That'd be weird.
     
  20. Oct 21, 2007 #19
    This has brought me to a new question, I've understood thus far everything, however. I read light doesn't have a mass, if thats true why can black holes bend light. Next if light does in fact have a mass then regardless of how insignificant, than does the wall or the person gain mass as they absorb light?
     
  21. Oct 21, 2007 #20

    Dale

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    The basic idea is that gravity doesn't directly bend light, rather mass curves spacetime. And then light just travels it's normal "straight" (geodesic) path through the curved spacetime.

    Yes. Since E=mc^2 when you absorb light you gain energy and therefore mass.
     
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