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Thread: Light

  1. #1 Light 
    Forum Sophomore Dkav's Avatar
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    Heres the scenario: a projector is hanging from the ceiling in a room and is about to project a movie onto a screen on the wall. If you record a video of that projector as it turns on, then replay that video in ultra-extremely-slow motion, is there any way you can see the light from the projector traveling through space and time (the room)???


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  3. #2 Re: Light 
    . DrRocket's Avatar
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    Quote Originally Posted by Dkav
    Heres the scenario: a projector is hanging from the ceiling in a room and is about to project a movie onto a screen on the wall. If you record a video of that projector as it turns on, then replay that video in ultra-extremely-slow motion, is there any way you can see the light from the projector traveling through space and time (the room)???
    no

    If all of the photons from the projector zre directed toward the screen then none of them will reach the video camera.


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  4. #3  
    Forum Sophomore Dkav's Avatar
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    What if you spray liquid nitrogen around the light, making it visible?
    I guess my main question is how come you cannot record light hitting a target in a dark room, then replay that tape in ultra slow motion to see the light slowly travel as you would for example and airplane?
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  5. #4  
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    Quote Originally Posted by Dkav
    What if you spray liquid nitrogen around the light, making it visible?
    That scatters the light. You still don't see the photons that go from the projector to the screen.
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  6. #5  
    Forum Sophomore Dkav's Avatar
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    Thanks for answering. However, I have more questions so bare with me...Is there a way that you can see those photons?
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  7. #6  
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    yes. The photons you see are the photons that travel towards you, and you only see them when they get to you. In short, the only thing you actually see are photons that happen to hit you're eye or the lens of the video camera if you are looking to record it. You can't see the photons that are about to hit your eye, only the ones that have hit your eye
    Wise men speak because they have something to say; Fools, because they have to say something.
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  8. #7  
    Forum Sophomore Dkav's Avatar
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    SOoo there is no way to see photons that are about to hit your eye from a source that is emanating light?
    The need for an intelligible world begins with the fearfulness of pre-philosophical, pre-literate societies facing an unpredictable world of change and trying to make sense of it.
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  9. #8  
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    Quote Originally Posted by Dkav
    SOoo there is no way to see photons that are about to hit your eye from a source that is emanating light?
    There is not, no. You get to see them when they hit your eye, but not before.
    Wise men speak because they have something to say; Fools, because they have to say something.
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  10. #9  
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    The need for an intelligible world begins with the fearfulness of pre-philosophical, pre-literate societies facing an unpredictable world of change and trying to make sense of it.
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  11. #10  
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    Quote Originally Posted by Dkav
    What about it ?
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  12. #11  
    Forum Sophomore Dkav's Avatar
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by Dkav
    What about it ?
    Ha I knew you would say that. Its a device that slows light to a crawl at room temperature, so theoretically this (slowing light down so much as to see it moving) would work right?
    The need for an intelligible world begins with the fearfulness of pre-philosophical, pre-literate societies facing an unpredictable world of change and trying to make sense of it.
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  13. #12  
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    Quote Originally Posted by Dkav
    Quote Originally Posted by DrRocket
    Quote Originally Posted by Dkav
    What about it ?
    Ha I knew you would say that. Its a device that slows light to a crawl at room temperature, so theoretically this (slowing light down so much as to see it moving) would work right?
    no
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  14. #13  
    Forum Sophomore Dkav's Avatar
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    Baloni!
    The need for an intelligible world begins with the fearfulness of pre-philosophical, pre-literate societies facing an unpredictable world of change and trying to make sense of it.
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  15. #14  
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    Quote Originally Posted by Dkav
    Baloni!
    wrong

    And the reason should be obvious by now.
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  16. #15  
    Forum Junior TheDr.Spo's Avatar
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    I'm with Arcane here on the need to understand that just because it is a photon does not mean you can see it. You have to understand how a human being is able to see. As Arcane pointed out, you see the photons that hit your eye. One that has not hit your eye yet has not made its existence known. The only way to see it is if it were scattering other photons toward your eye (or the lens in question).

    The way that light slows down in a material is a different matter altogether. The light isn't slowing down in the literal sense that a driver would apply brakes to his moving car. The light is making stops, like a driver stopping at a rest area along his journey from point A to point B. I believe this metaphor might be overused but... eh, what the hell.
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  17. #16  
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    Quote Originally Posted by TheDr.Spo
    I'm with Arcane here on the need to understand that just because it is a photon does not mean you can see it. You have to understand how a human being is able to see. As Arcane pointed out, you see the photons that hit your eye. One that has not hit your eye yet has not made its existence known. The only way to see it is if it were scattering other photons toward your eye (or the lens in question).
    Precisely. And photons don't interact with photons directly.
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  18. #17  
    Forum Sophomore Dkav's Avatar
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    Quote Originally Posted by TheDr.Spo
    I'm with Arcane here on the need to understand that just because it is a photon does not mean you can see it. You have to understand how a human being is able to see. As Arcane pointed out, you see the photons that hit your eye. One that has not hit your eye yet has not made its existence known. The only way to see it is if it were scattering other photons toward your eye (or the lens in question).

    The way that light slows down in a material is a different matter altogether. The light isn't slowing down in the literal sense that a driver would apply brakes to his moving car. The light is making stops, like a driver stopping at a rest area along his journey from point A to point B. I believe this metaphor might be overused but... eh, what the hell.
    Word.
    The need for an intelligible world begins with the fearfulness of pre-philosophical, pre-literate societies facing an unpredictable world of change and trying to make sense of it.
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  19. #18  
    Forum Sophomore Dkav's Avatar
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    Do you guys have PhD's?
    The need for an intelligible world begins with the fearfulness of pre-philosophical, pre-literate societies facing an unpredictable world of change and trying to make sense of it.
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  20. #19  
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    Dkav: you can't actually see light, you use it to see. I know that sounds a little odd, but if the sky is clear tonight step outside and look up at the moon. You can see it because light from the sun hits the moon and is reflected back into your eye. Then ask yourself this what can I see?. You don't actually see the light, what you see is the moon. Now look at the space beside the moon. There's light travelling through that space, and a lot of it. But you can't see it. All you see is blackness. If I shone a powerful laser beam through empty space you wouldn't see that either. If however there were dust particles in that space, you'd see a laser beam. If the region of space was big enough you'd see the laser beam extending out from the laser, but at all times you're using the light that is reflected into your eye to see the things that the light lights up.

    Doc: see http://en.wikipedia.org/wiki/Two-photon_physics. The interaction is described via virtual particles, but those particles are virtual rather than short-lived real particles.
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  21. #20  
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    Quote Originally Posted by Farsight
    Doc: see http://en.wikipedia.org/wiki/Two-photon_physics. The interaction is described via virtual particles, but those particles are virtual rather than short-lived real particles.
    I am aware of this phenomena, which is why I was careful to state that photons do not interact directly with one another (because they carry no electric charge). This interaction involves QCD rather than just QED.

    The point, for the purpose of the question at hand, is that photons are not illuminated by other photons -- you don't "see" photons, but rather detect photons.
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