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The physics of colour

  1. Jul 20, 2007 #1
    Hi,

    A few days ago, during a discussion at my chemistry class, I suddenly realised something very fundamentally puzzling about colour. We say that if in a molecule (say a conjugated organic system like beta-carotene) the HOMO-LUMO gap corresponds to a visible frequency of light, we observe that compound to be coloured because that frequency is absorbed and we see the complementary colour.

    But consider this: when equilibrium has been established between the ground state and he excited state, the number of molecules getting excited in unit time is the same as the number getting deexcited. This should mean that the same amount of light being absorbed is also being emitted. If this were not true, then molecules would accumulate in the excited state.

    This seems to imply that coloured compounds would decolorise after a short exposure to white light. While light does very gradually decolourise substances, it is due to photochemical decomposition and oxidation of dyes rather than reaching equilibrium. What is the explanation? Thanks.

    Molu
     
  2. jcsd
  3. Jul 20, 2007 #2

    Dick

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    I guess this observation would imply there are other modes for a molecule to deexcite besides the extremes of reemittng the same wavelength and disintegrating.
     
  4. Jul 20, 2007 #3
    You mean that an electron excited to the previously-LUMO has other options than dropping back into the now-singly-occupied HOMO? And these options don't involve radiation? That seems too convenient...

    Molu
     
  5. Jul 22, 2007 #4
    Anyone to help?
     
  6. Jul 24, 2007 #5
    Anyone to help?
     
  7. Jul 24, 2007 #6

    mgb_phys

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    Consider the solid angle that light is being emmitted into compared to the small solid angle of the light that is being reflected to your eye.
     
  8. Jul 25, 2007 #7
    But even within my viewing cone, the amount of light absorbed should be equal to the amount of light emitted. This seems like such an obvious question, yet I can't find the answer!

    Molu
     
  9. Jul 29, 2007 #8
    Anyone to help?
     
  10. Jul 29, 2007 #9

    Dick

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    Your optically active molecules are in a complex environment with various sorts of interactions with neighbors. But you seem to have decided that the only way it can change state doesn't involve these. Maybe it's time for you to explain why you think it MUST involve radiation.
     
  11. Jul 30, 2007 #10
    You are merely speculating. What is the actual explanation?

    Molu
     
  12. Jul 30, 2007 #11

    Dick

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    The explanation is that the molecule can deexcite thermally by interacting with neighboring molecules - not only radiatively. I was waiting for you to realize this yourself.
     
    Last edited: Jul 31, 2007
  13. Aug 2, 2007 #12
    But why do photo-excitted molecules prefer to deexcite thermally rather than radiatively? And do you know this to be the explanation (i.e. have you read it in some reputable published source) or are you simply advancing a plausible hypothesis?

    Molu
     
  14. Aug 2, 2007 #13

    Dick

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    I know for a fact if I put a colored solution in sunlight that it doesn't instantly turn colorless from saturation or bleach from photodisintegration, it gets hot. I didn't feel the need to seek out a reputable published source.
     
  15. Aug 5, 2007 #14
    Alas! If theories of science could truly be verified that easily...

    Molu
     
  16. Aug 5, 2007 #15
    EDIT: post removed
     
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