Is it a valid question to ask what is the percentage of light that gets refracted by H 2 for a certain distance(Lets say 1 light year) in deep space. Lets assume a few(3) Hydrogen atoms per cubic meter.

  • $\begingroup$ Sounds like a valid question to me... the astronomers might have a more "handy" answer for you, though. The problem I see for atomic hydrogen is that it has a really, really low cross section for frequencies that aren't within the line width of one of its transitions and the spectrum of molecular hydrogen is remarkably complex... I simply wouldn't know which of the many lines is of astronomical importance or if the way to "see" hydrogen clouds is by using a coarsely averaged spectrum and how to get that from first principles with absolute units... $\endgroup$
    – CuriousOne
    Jan 10, 2016 at 0:31
  • $\begingroup$ @CuriousOne, does 1.06 x10^-10 meter sound correct for collisional cross section for H 2. $\endgroup$ Jan 10, 2016 at 0:44
  • $\begingroup$ BTW, Would the back of the envelope answer "It's roughly given by the index of refraction of hydrogen at normal conditions corrected for the density of space (a factor of roughly 4e-25, I believe)." work? In the visible that brings us to $n\approx 1 + 5*10^{-30}$. $\endgroup$
    – CuriousOne
    Jan 10, 2016 at 0:44
  • $\begingroup$ @CuriousOne, that does make good sense your line of thinking. $\endgroup$ Jan 10, 2016 at 0:51
  • $\begingroup$ Tho, its a dimensionless number, so thinking how I can apply it in answer. $\endgroup$ Jan 10, 2016 at 0:55


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