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Why does a black body radiate in all the frequency spectrum?

  1. Jan 18, 2016 #1
    I understand why a black body absorbs every frequency(it is the definition of a black body!) but i do not understand why it also radiates at all frequency spectrum.
     
  2. jcsd
  3. Jan 18, 2016 #2

    DaveC426913

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    Is there a reason to expect it would not radiate at certain frequencies?
     
  4. Jan 18, 2016 #3
    Hello. No, but my question is why radiating at all frequencies means also absorbing at all frequencies and vice versa. Why does the one mean the other?
     
  5. Jan 18, 2016 #4

    DaveC426913

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    According to Wiki:
    https://en.wikipedia.org/wiki/Black_body#Cavity_with_a_hole

    For the same reasons, it would also be a nearly perfect emitter.
     
  6. Jan 19, 2016 #5

    Demystifier

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    The correct statement is that black body in thermodynamic equilibrium radiates at all frequencies. In principle, a black body might not be in a thermodynamic equilibrium, in which case it might not radiate at all frequencies.

    If you want to ask why thermodynamic equilibrium implies radiation at all frequencies, it is because thermodynamic equilibrium, by definition, is a state with maximal entropy.
     
  7. Jan 19, 2016 #6
    About your last statement, why having maximal entropy means radiating in all frequencies?(i have not taken a course on thermodynamics). Thank you for the answer by the way
     
  8. Jan 20, 2016 #7

    Demystifier

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    Entropy can be thought of as the number of different possible states. Clearly, there are more possible states if all frequencies are allowed than if only some of them are allowed.
     
  9. Jan 22, 2016 #8
    Emission and absorption of photons are both reverse reactions to each other. They are pretty much the same thing, just reversed in time. Any time you have one, you have the other. This is governed by the thermodynamic principle of detailed balance.

    Consider an object behind some kind of hypothetical cavity box that only allowed a single wavelength of light through. If the object could emit at that wavelength but could not absorb at that frequency, the object would eventually emit all its energy away to the surroundings, even until the object is colder than the surroundings. This is not allowed by thermodynamics. So the object must also absorb at that same wavelength.
     
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