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Black body re-emission of incident radiation

  1. Apr 30, 2015 #1
    I know that a black body is an idealized body that absorbs all the incident EM radiation. And I also know that the BB emits a spectrum of thermal radiation that depends on its temperature.

    What I cannot understand is the re-emission. For what i've read, when in thermal equilibrium, a BB absorbs and re-emits continuously all the incident radiation. Is that right?

    So, when in thermal equilibrium, if a black body absorbs visible light, then why doesn't it re-emit visible light?
  2. jcsd
  3. May 1, 2015 #2


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    But it does emit visible light. Every hot glowing body does.
  4. May 1, 2015 #3
    Yes, but my doubt is for the light that it absorbs. Say that a BB is in thermal equilibrium at room temperature, and there is white light inciding on it. Why doesn't the BB re-emits this white light? Where does it go? Does it raise the temperature of the BB?

    Obs.: Sorry for my bad english :)
  5. May 1, 2015 #4


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    Yes, the white light is absorbed. But, as the BB has a lower temperature as the white light source, the white light will be absorbed and eventually rise the temperature of the BB. However, the BB will only emit the spectrum corresponding to it's temperature, which has it's maximum in the far infrared. Absorption and emission are two completely independent processes in a BB.
  6. May 1, 2015 #5
    Ok, now I understand. Thank you :)
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