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Light and Photons

  1. Sep 24, 2004 #1
    Hi Guys,

    I`m having a bit of trouble understanding photons, and have thought up of a scenario that may clear up my thinking on it.

    First of all, imagine a completely dark room (say, one made completely out of concrete, with no doors or anything, so light can`t sneak in through keyholes etc). I am standing at one side of the room, and on the other there is an object (say a red car), which I can`t see. Why can`t I see it?? Is it because the car isn`t emitting any photons, or that it is, but I just can`t see them.

    I remember reading a similar post somewhere (not sure if it was on this forum or not) where somebody replied that I couldn`t see the car as my eyes weren`t used to the darkness or something, and that after a while, after my eyes had adjusted it would become visible. This doesn`t make sense to me, as I would have thought that this could only happen if there was at least *some* light in the room, which there isn`t (remember, no keyholes!!!)

    Now, I take a torch and shine a beam of light at the car. Now I can see it, what has changed?? Is it that the light from the torch is being reflected off the car back to me, or that the light 'stimulates' the car, so that it emits photons back to me (and presumably in every direction).

    help would be appreciated.....
  2. jcsd
  3. Sep 24, 2004 #2
    Well, in a completely dark room, there are no visible wavelength photons present, so you can't see a thing.

    (there could be IR, gamma rays or whatever)

    A torch produces light across the whole visible spectrum. When you see an object, you see reflected light. The red object reflects photons with the appropriate wavelength - red ones.
  4. Sep 24, 2004 #3


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    Actually, there are visible wavelength photons! Your eyes just aren't sensitive enough to see them - no matter how long you wait for your eyes to adjust.
  5. Sep 24, 2004 #4
    thanks for the replies

    So, when the torch is turned on the light (photons) from the torch go to the red car. They hit the car and reflect back to my eyes in the form of photons of a particular wavelength correcponding to red light. Is this right?? Are they the same photons emitted from the torch, or are they 'new' ones generated by the car??

    I have a bit of confusion over the difference between reflecting photons (the car, or anything else), and producing photons (the torch/the sun etc.)

    Thanks again
  6. Sep 24, 2004 #5


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    Yes, when you shine your torch the very same photons come back, strike your retina and you see!

    Generally, "new photons" come from incandescence, phosphorescence or fluorescence or some similar process.
  7. Sep 24, 2004 #6


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    Although there are many who would say that "reflection" involves the absorption of the photons by the reflecting surface, which then emits photons. Whether those are the "very same photons" is open to interpretation!
  8. Sep 24, 2004 #7
    Incandescence phosphorescence or fluorescence don't necessarily emit the same way they absorb (absorption of two low frequency photons => emission of a high frequency one for example).

    With reflection, what is emitted is identical to what is absorbed (except for direction).

    When you shout and there is an echo, do you say that the reflected voice is the "same" voice" or a "new" voice?
    Last edited by a moderator: Sep 24, 2004
  9. Sep 24, 2004 #8
    "I would say "new", caused by wall vibrations."

    Wouldn't a wall with infinite mass and stiffness (one that couldn't vibrate or move) still reflect sound waves though?
  10. Sep 24, 2004 #9
    Good point, I edited that out. AFAIK, infinite wall mass means perfect reflexion with a 180 phase shift.
  11. Sep 24, 2004 #10


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    Of course, if the car were hot (not just the 'Gone in 60 seconds' kind of hot!), you could see it with your own eyes, just as you can see an incandescent light globe, or an electric space heater, or a poker pulled from the fire, or ...

    (mind you, it may not be a good idea to try to drive a car that's hot enough to see by its own thermal radiation!)
  12. Oct 12, 2004 #11
    Thanks for the answers, but......

    Does that Thermal radiation you mention occur due to the atoms shaking about so much (as the car is really hot) that the atoms start to emit photons??

    So, If I`m getting this correctly, when I shine a photon at a car, the photon hits the atoms in the car, and shakes them about a bit (obviously not the technical term!!!), which makes them emit a 'new' photon. - This is reflection -- Correct??

    Wheras, if a car gets really hot, the atoms are moving about so much that they emit photons all by themselves (and the hotter they get, the more they move around which changes the wavelength which enables the radiation to be red hot, or white hot etc??) --this is emmision of light. -- Correct??

    Am I off the mark here???


  13. Oct 13, 2004 #12
    If the photons are of a visible wavelength, why aren't they visible?

    Or do you mean that humans can't detect them but some animals can?

    Visible light implies that humans can see it, no?

    Afterall, all EM radiation can be detected somehow, whether it's by a mechanical device or by an evolved biological specimen such as a human.
    Last edited: Oct 13, 2004
  14. Oct 13, 2004 #13
    Close. As the car gets hot, the atoms do "shake about". This causes the atoms to jump to a higher state and then -- when they drop to a lower state -- they radiate photons. This is known as blackbody radiation.

    Here is a link that talks about it.
    Again you are describing blackbody radiation, and your understanding is basically correct.

    As discussed in the link that I gave, a room-temperature object will emit visible light, but the amount of visible light is so low that our eyes cannot detect it even when dark-adapted. As you heat the object, a larger about of light will be emitted in the visible and the object will start to glow.
  15. Oct 13, 2004 #14


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    I meant simply that there aren't enough of them to fire the rods and cones on your retina and produce a neural response adequate to perceive them.
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