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Why do light disappear?

  1. Jun 5, 2012 #1
    Hi, Im new to physics, so this might be a stupid question.

    Imagine a pitch black room. If you open the door and shoot water into the room and then close the door, the water will still be in the room.

    If you do the same thing, but this time with light instead of water. How come the light disappear from the room when you close the door? Why isn't the photons caught in the room?
  2. jcsd
  3. Jun 5, 2012 #2
    The light hits the wall and get absorbed by the wall.
  4. Jun 5, 2012 #3


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    Not forever! Eventually it will be absorbed into the air, floor and walls.

    They are absorbed into the air, floor, and walls, like the water, just much faster.
  5. Jun 5, 2012 #4


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    More specifically, when photons encounter matter, they tend to get absorbed, transferring their energy and heating up the matter. So, once the door is closed, the room will be slighter warmer than before. That's where the photons have gone.
  6. Jun 5, 2012 #5
    Interesting thread, "This space for rent" I love that. Good play on words.

    I observed a falling star and it appeared seemingly from no where into a long streek and then disapeared. Knowing that this light had traveled a milenia of years to reach my eye and, in this case, that light has a begining and an end. Or at the very lease saw it end. All of that energy that I saw distiguised was absorbed or as you say the matter around it absorbed it along its way, the energy would be disapated in all directions from its' source. Or am I way off in space, that is rentable, here? :)
  7. Jun 5, 2012 #6


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    No, the light that reaches your eyes is emitted a tiny fraction of a second before it hits your eyes.
  8. Jun 5, 2012 #7


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    "a falling star" is actually a meteorite that hits the Earth's atmosphere and burns up - only a few hundred km from where you are looking. The short burst of light is only generated a few milliseconds before it reaches you.

    The difference between the light entering a room and water, sprayed from a hose, is that the light is just energy and the water is matter. However, the Kinetic Energy (motion) that the water has when sprayed into the room soon dissipates itself and the water will come to a halt so, in both cases, energy soon dies down and is absorbed into the substance of the room (even is the walls don't actually 'absorb' the water).
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