The collective properties of a plasma, in its interaction with a radiation field, become important when the electron plasma frequency is comparable with, or exceeds, the radiation frequency. The particle interactions must properly be treated quantum mechanically when electron plasma wave phonon energies are of the order of, or greater than, average random electron energies. Both of these conditions may be satisfied for the system of optical or infrared radiation interacting with a dense plasma. Expressions for the high frequency conductivity and opacity are obtained for a dense, fully ionized plasma. It is found that, when the photon energy greatly exceeds the average random energy, and when the electron plasma frequency is much greater than the radiation frequency, then the free‐free contribution to the opacity, ordinarily written without the inclusion of collective effects, is reduced by a factor of the order ofE¯&OHgr;3/ℏ&ohgr;p4, whereE¯is the mean random electron energy in the plasma, &OHgr; is the radiation angular frequency,ℏis Planck's constant, and &ohgr;pis the electron plasma frequency.