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leonmate
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Homework Statement
Consider photons crossing a large hot ionised hydrogen region with constant electron density ne = 5*105 m-3. Assuming that the photons and the electrons interact via Thomson scattering (with Thomson scattering cross section, σT = 6.65*10-29 m2, what is the thickness Δx that this region should have in order to be 'optically thick'?
Homework Equations
Sifting through my textbooks I've found a few useful equations:
Optical depth for the medium for Thomson scattering:
τ = ∫ σT * ne dx
Mean free path:
l = (σT * ne)-1
The Attempt at a Solution
So, I've thought about this problem a couple of ways. The main issue is I can't find a definition for 'optically thick'. At what point does the gas region become optically thick. Is it when it is larger than the mean free path? I've also got a few notes about self-absorption processes in optically thick mediums, I don't know how to make that relevant either.
Another note, I've also read:
'For a completely ionized hydrogen gas, ne is equal to the number density of protons that carry most of the mass'
from my textbook: The Physics of Astrophysical Processes, Bradt.
So, if this is the case then I need to swap out my Thomson cross section as this is related to the mass of the particles and it would now be ~10-35 m2.
This increases the mean free path significantly also, from 3*1022 up by a factor of 106
Any hints would be greatly appreciated!