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Homework Help: Help needed! Mean free path of photons.

  1. Feb 16, 2008 #1
    1. The problem statement, all variables and given/known data
    Neutral atomic hydrogen cloud, density [tex]n_H[/tex]. Absorption cross section [tex]\sigma_{0} = 6.3*10^{-18}cm^{2}[/tex]. Determine the mean free path of photons with energy of 20eV, for densities [tex]n_H[/tex] = 1, 10 and 100 [tex]cm^{-3}[/tex]. Compare this to mfp for photons with ionisation energy of hydrogen (13.6eV) at the same densities.

    2. Relevant equations

    The only possibly relevant equation given in our notes is [tex]\tau_{\nu}=\sigma_{0}*(\frac{\nu}{\nu_{0}})^{-3.5}*n_{H^{0}}[/tex]

    3. The attempt at a solution

    I'll be able to try the second bit once I figure out the first bit! I looked up some stuff on the mean free path, and figured that it could've just been [tex]l=\frac{1}{n_{i}*\sigma}[/tex], but then is [tex]n_i[/tex] the same as [tex]n_H[/tex]? And the equation given in the notes (above), all that is said about it is that the optical depth of photons above the Lyman limit [tex]\nu_0[/tex] can be derived from it. I simply can't see where to put in the amount of energy so that it will make a difference!
  2. jcsd
  3. Feb 18, 2008 #2
    Anyone got any idea?
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