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Thermal radiation

  1. Aug 1, 2011 #1
    I read somewhere on the net that almost 100 percent of radiation in the universe is thermal.

    What then is nonthermal radiation? Can it be said that blackbody radiation is purely thermal?

    What about all the radiation that is reflected, diffracted or partly transmitted by bodies, is that nonthermal radiation?

    I also read that emission of photon by atoms when they jump into lower states is thermal radiation. How can this be? That has nothing to do with heat.

  2. jcsd
  3. Aug 1, 2011 #2


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    Rigorously speaking, "thermal radiation" should be taken to mean, "Radiation from an ensemble of quantum states that is described by a Boltzman distribution" (see http://en.wikipedia.org/wiki/Boltzmann_distribution). It has *nothing* to do with heat per se, but instead only has to do with the statistical behavior of quantum systems defined by a given temperature. The frequencies of the radiation will depend on the spacing between the quantum states. The temperature will determine the relative populations of the quantum states, which are assumed to be in thermal equilibrium.
  4. Aug 6, 2011 #3


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    Thermal radiation is black body radiation. Nonthermal radiation is anything else. Light from a filament light bulb is thermal, while light from your LCD monitor is nonthermal. Light from the sun is themal (mostly), while light from aurora borealis is not. Light reflected by an opaque object loses its thermal nature since some wavelengths are reflected more strongly than others.
  5. Aug 6, 2011 #4


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    Please cite ALL sources. https://www.physicsforums.com/blog.php?b=2703" [Broken].

    Last edited by a moderator: May 5, 2017
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