Transition radiation of photons and neutrinos at a plasma boundary

It is shown that photons and neutrinos with nearly constant velocity can radiate a large spectrum of electromagnetic waves when they cross the boundary between a plasma and vacuum. This is a direct consequence of the existence of an equivalent electric charge of photons or neutrinos in a plasma. The general expression for the radiation field in the vacuum region far away from the boundary is established, and the particular case of a sharp plasma boundary is discussed.