On Transition Fluxation*

A treatment of transition radiation between two dielectric media is presented which is based on the exact expressions for the fields of the particle in the two media. Ex-*-pressions for the spectral distribution of the energy emitted foreward and for that emitted backward are derived. The results are in accord with experimental findings for ultra-relativistic particles. It is indicated how the treatment can be extended to _ the case of a plate and to that of a wave-guide, as well as to emission by a monopole. The case of the simultaneous emission of transition radiation and Cerenkov radiation is considered and the relationship between them is clarified. It is also found that the Cerenkov wave emitted by the particle in the backward medium will be partially reflected , and partially refracted into the foreward medium, after the particle crosses the boundary between the two media. The linear energy density for the refracted wave is calculated and it is shown that under certain feasible conditions this is amenable to experimental verification.