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Homework Help: Blackbody radiation and Planck's distribution

  1. Jul 8, 2011 #1
    physics books on the topic of blackbody radiation say that any given object when heated will emit a continuous electromagnetic radiation which depends only on temperature. Planck's distribution based on discretization of energy states accurately describes this phenomenon.
    But is this true?
    If I put some hydrogen gas in a container and heat it up, will not it emit light at specific frequencies and not continuously?
  2. jcsd
  3. Jul 8, 2011 #2


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    There is a small flaw in your reasoning: The classical 'black body' radiator is a solid substance. Hydrogen gas, or any other gas, is not a solid body.
  4. Jul 9, 2011 #3


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    That's not quite correct. The assumption that leads to the blackbody spectrum is just that you have an (approximate) continuum of quantum states over the range of the energies that can be significantly excited by the thermal environment. A gas can be a perfectly fine approximate blackbody over a range of temperatures. It's when the temperature gets into the range where you can excite the ionization spectrum of the gas (which can be VERY noncontinuous) that the blackbody approximation breaks down. But even then the sun is a pretty good blackbody, and it's a gas.
    Last edited: Jul 9, 2011
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