1. The problem statement, all variables and given/known data 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'? 2. Relevant 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 3. 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 text book: 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!