I am trying to form some intuitive concept about the free-free absorption coefficient for a hydrogen plasma. The standard expression (cf. e.g. Schwarzschild's Structure and Evolution of the Stars, 1958, p64) contains a reasonably familiar particle type cross-section component, but there is also a velocity term in the denominator. This velocity term means that the transparency rises (absortpion goes down) as the temperature rises ( a \sqrt(T) term in the denominator). This might seem reasonable, I think, if it meant, for example, that the Debye length decreased with this temperature term. Basically, then a photon would have less chance to see a proton with nearby electron pair with which to exchange some energy, because there would be less chance to find the electron in a suitable, interactable, state. But the Debye length in a plasma increases with just that same \sqrt(T) dependence -- so this idea cannot be right. Another possibility might be a relativistic foreshortening effect on the cross-section which will increase with increasing transverse velocity. But there is no mention of 'relativistic effect' when one comes across the alpha_ff formula. The root question, I suppose, is, where does the alpha_ff formula come from? I have not found it sourced in the books at my disposal. Any suggestions?