Coherent Thomson scattering by cold electrons changes a photon's direction without changing its frequency in the electron rest frame. A monochromatic incident radiation field therefore remains monochromatic after arbitrarily many such scatterings. Large scattering optical depth can isotropize radiation but does not imply a Planck photon distribution. True absorption and emission, or suitable additional inelastic processes and photon production, are needed to establish a blackbody spectrum independently of the initial radiation field.
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