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How does matter absorb light?

  1. Apr 7, 2012 #1
    How does matter "absorb" light?

    I can not find anything on how matter "absorbs" light and what it does when it does. Can anyone explain this?
     
  2. jcsd
  3. Apr 7, 2012 #2

    haruspex

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    Re: How does matter "absorb" light?

    there are several ways. All involve a net acceleration of charge.
    A. Electron bumped to higher energy level, in the extreme case, ionisation, being decoupled from the atom. In this case the electron will drop back down, re-emitting light, but if ionisation occurred the new photon may be rather lower in energy. Even in the non-ionising case, I believe it may be of slightly lower energy because of Doppler effects.
    B. Vibration of an ionic bond or intramolecular bonds in a polar molecule - e.g. H2O or CO2 rather than N2 or O2. In this case it may again be re-emitted or perhaps absorbed by intermolecular collisions.
    C. Impacting and accelerating an ion. This is why plasmas are effectively opaque - they absorb and re-emit light at all frequencies.
    Can't think of any others.
     
  4. Apr 7, 2012 #3

    mfb

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    Re: How does matter "absorb" light?

    Well, it depends on the frequency of the light.
    I would like to add one important thing for visible light: In metals, there are electrons which are not bound at a single atom. They can absorb photons easily, and usually re-emit them afterwards with the same frequency. That is the reason why metallic surfaces reflect light.

    For other frequency ranges (not visible light), there are more options how the photon can react with matter.
     
  5. Apr 7, 2012 #4
    Re: How does matter "absorb" light?

    This question touches quantum fundamentals 'spontaneous emission'. The event of emission or absorption of light/radiation cannot be comprehended like classical continuous mechanism. if it is possible we can stop the radiation process in the half stage, then only half wave length of photon will be emitted. Funny know? However it is not like that. I think nobody could satisfy, for this kind of question. Even top-most scientists knows only the probability of radiation or absorption to occur for the given situation.
    see: wikipedia
    We cannot say anything about particular photon will come like this, will do like this and will go like this. Then how does all these telecommunications works, its all about 'technical know how'. Nobody completely knows why it is the way it is? We do just pass the current/single in one dish and get it at the other dish, that's all. It works, what else we need?
     
  6. Apr 7, 2012 #5

    mfb

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    Re: How does matter "absorb" light?

    And quantum theory shows that this is the best result you can get. Probablity is not due to our lack of knowledge, it is a fundamental property of the universe (with some different interpretations, but the main message is the same in all).

    The question "why" can be translated to "why are the fundamental laws of nature like they are?". And this is a question without a good (!) answer.
     
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