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Homework Help: Planck Black-Body Law

  1. Jan 31, 2014 #1
    1. The problem statement, all variables and given/known data
    Starting from the Planck-Body Law

    I[itex]_{λ}[/itex]dλ = [itex]\frac{2\pi c^{2}h}{λ^{5}}[/itex] [itex]\frac{1}{e^{hc/(λkT)} - 1}[/itex]dλ

    where λ is the wavelength, c is the speed of light in a vaccuum, T is the temperature, k is Boltzmann’s constant,
    and h is Planck’s constant, prove that the total energy density over all wavelengths is given by

    I[itex]_{tot}[/itex] = aT[itex]^{4}[/itex]

    and express a in terms of pi,k,h,c
    2. Relevant equations
    λ = c/f

    3. The attempt at a solution
    Our teacher gives us a hint "think about whether it is better to do the integral in the wavelength or frequency domain" - which in this case means he wants us to switch to the frequency domain. I did try a bunch of things but I am just not sure if my first step is correct. To switch to the frequency domain, all I havr to do is plug in

    λ = c/f
    dλ = -c/f[itex]^{2}[/itex]

    correct? Or is this first step wrong
    Last edited: Jan 31, 2014
  2. jcsd
  3. Feb 1, 2014 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    dλ = -c/f2 ... there should be a df in there somewhere.

    [tex]\renewcommand{\d}{\;\text{d}}[/tex] [tex]I_\lambda \d\lambda = \frac{2\pi c^2 h}{\lambda^5}\frac{\d \lambda}{e^{hc/\lambda kT}-1}[/tex] ... can you see why it may be easier to change to frequency domain?

    Note: when a hint says to "think about" something, you usually get extra marks for showing that you actually thought about it instead of just taking the hint blindly. Sometimes a teacher will hive you a false "think about" in the hint and you are supposed to dismiss it with reasoning. Therefore: check that the hint makes sense.
    Last edited: Feb 1, 2014
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