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How does a blackbody absorb radiation? and why?

  1. Aug 2, 2009 #1
    I've read that the phenomenon of black-body radiation was one of the key factors in the creation of quantum mechanics. So I want to learn about it...;-)

    First of all I don't understand why the the temperature of the black body increases when it absorbs energy from photons.
    Why can't the electrons that were excited (when the photon was absorbed) simply jump down to their original energy level and release the excess energy as a photon? (similar to how atomic spectra are formed)

    (These questions occurred to me because I assumed that the light is absorbed by excitation of electron. Please correct me if I'm wrong... :-)

    Thanks.,
     
  2. jcsd
  3. Aug 2, 2009 #2

    sylas

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    Light is also absorbed as it gives vibration energy to the molecules. The excitation of electrons requires higher energies, and is associated with the photoelectric effect. Interactions with thermal radiation is mainly with atoms and molecules as small oscillators, which is a different process.
     
  4. Aug 2, 2009 #3
    How does light give vibration energy to the molecules?
     
  5. Aug 2, 2009 #4

    ZapperZ

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    You may want to start by reading an entry in the FAQ thread in the General Physics forum. Pay attention to the entry on photons moving through a solid material.

    Zz.
     
  6. Aug 2, 2009 #5

    sylas

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    Molecules in most materials are held together by bonds involving some kind of electromagnetic interaction. On atomic scales, therefore, molecules involve distributions of charge; vibrations or oscillations are accelerations of charge, which interact strongly with electromagnetic radiation.

    I have been trying to learn about interactions of radiation and gas in recent times. Some gases interact very poorly with radiation; but molecules like H2O, or CO2, or N2O, or more complex molecules, have various natural modes of oscillation; and the molecules are dipoles or the like because the electrons of the co-valent bond tend to be closer to one of the atoms than the other. These vibrations are the basis for absorption and emissions of thermal radiation.

    Cheers -- sylas
     
  7. Aug 2, 2009 #6

    ZapperZ

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    Note that in blackbody radiation, it is typically the interaction of light on a solid object, not on gasses.

    Zz.
     
  8. Aug 2, 2009 #7
    Interestingly, stars and black holes are also black bodies according the Leonard Susskind, THE BLACK HOLE WAR, 2008. (This means both are excellent absorbers of electromagnetic radiation.)

    He also says, page 171,referring to more prosaic materials as water or air:

    On page 171 he provides a Feynman diagram of such phenomena.
     
  9. Aug 2, 2009 #8

    sylas

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    Good point; I agree. A gas is rarely a blackbody radiator. Typically, solid bodies are not pure blackbodies either; although they are usually closer. But the Sun is pretty close to a blackbody.

    I take your point; but I was really aiming to answer the question of physical mechanisms for thermal radiation more generally, which is not about the excitation of electrons in and out of different orbitals of an atom.

    Cheers -- sylas
     
  10. Aug 2, 2009 #9

    Andy Resnick

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    One minor point- "blackbody radiation" refers to a specific spectral distribution of an electromagnetic field; the material constitution of the cavity (or object) itself is of no consequence. A "photon gas" is at thermal equilibrium, with a specific temperature 'T', when it has the properties of blackbody radiation.
     
  11. Aug 2, 2009 #10
    So here's a short summary of what I understood..
    black body radiation is dependant on the temperature of the body and nothing else. It is caused by the vibrational energy of the molecules of the object.
    In that case, is black body radiation unrelated to the fact that blackbodies can absorb all frequencies? If black body radiation is dependant on vibrational energy and absorption of EM waves is dependent on the promotion of electrons then they should be unrelated right? (That's a genuine question.. I'm not saying they ARE related and that your explanations are wrong)
     
  12. Aug 3, 2009 #11

    f95toli

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    It is not really clear what you are asking.
    You can't really talk about specific mechanisms for absorption/emission from blackbodies simply because they don't exist in the real world.
    A blackbody is -as Andy Resnick has already pointed out- a mathematical construct, an idealized "object".
    There are many objects that behave ALMOST as if they were blackbodys (at least in a a certain frequency range) meaning it is a very useful theory.
    However, the theory does not specify WHY these objects behave like blackbodies and the underlying mechanisms differs from case to case.
     
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