  The problem of thermalization of radiation within a self-emitting hydrogen
isothermal atmosphere is considered for the case of a hydrostatic profile of
the plasma density. The probabilistic approach to define the thermalization
depth for the photon of a given frequency is used. Quantitative conclusions are
made on the value of the optical depth at which the radiation can be viewed as
thermalized up to a given frequency. The analytical and numerical solutions of
the radiative transfer equation confirm the obtained results. The frequency
dependencies of the probability of free-free absorption of a photon due to
single interaction in a cold nonmagnetized and magnetized plasma are
calculated.
