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Homework Help: Need help with blackbody radiation

  1. Jan 24, 2005 #1
    Can someone help me with this please?

    B(f) = [(2hf^3)/c^2]{1/[exp (hf/kT) - 1]} - Planck law in terms of frequency.

    "Derive expressions for B(f) for the cases i. hf << kT, ii. hf >> kT".

    I've done the first bit, that's just using a Taylor expansion. The second bit is where I'm stuck. In my lecture notes, I have that for hf >> kT, B(f) = [(2hf^3)/c^2]exp(-hf/kT). What I don't understand is why it's like that. If hf >> kT, shouldn't that exponential tend to infinity?

  2. jcsd
  3. Jan 24, 2005 #2


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    Yes,the exponential should much larger than 1 and therefore the 1 in the denominator would be neglected...The exp.would come on top with a "-" in its argument and from the Planck distribution,u'd be getting the Wien distribution...

  4. Jan 24, 2005 #3
    OH! Thanks :smile:.
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