Black body re-emission of incident radiation

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Discussion Overview

The discussion revolves around the concept of black body radiation, specifically focusing on the re-emission of incident radiation when a black body is in thermal equilibrium. Participants explore the relationship between absorbed visible light and the emitted spectrum based on temperature, addressing the mechanics of absorption and emission processes.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • One participant states that a black body absorbs all incident electromagnetic radiation and emits thermal radiation based on its temperature.
  • Another participant asserts that hot glowing bodies do emit visible light, suggesting that black bodies also emit visible light when heated.
  • A participant questions why a black body does not re-emit the visible light it absorbs when in thermal equilibrium, asking about the fate of the absorbed light and its effect on the black body's temperature.
  • In response, it is noted that while the black body absorbs white light, it will only emit radiation corresponding to its own temperature, which typically peaks in the far infrared, indicating that absorption and emission are independent processes.

Areas of Agreement / Disagreement

Participants express differing views on the nature of re-emission in relation to absorbed visible light, with some confusion remaining about the processes involved. The discussion does not reach a consensus on the specifics of how absorbed light affects the emission spectrum.

Contextual Notes

The discussion highlights the complexity of the absorption and emission processes in black bodies, with participants acknowledging the dependence on temperature and the independence of these processes. There is uncertainty regarding the implications of absorbed light on the black body's temperature and emission characteristics.

Julianotcp
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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?
 
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But it does emit visible light. Every hot glowing body does.
 
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 :)
 
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.
 
Ok, now I understand. Thank you :)
 

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