ABSTRACT
Transition radiation emitted when a source crosses a clear-cut interface is the simplest case. Generally, transition radiation is generated whenever a source (charge) moves uniformly in an inhomogeneous or j and a non-stationary medium or near it. Apart from the previously discussed 'annihilation' of a source and its image, transition radiation may be interpreted in a different very general fashion. This can be illustrated using an isotropic transparent medium with the refractive index n. Then, in the general case, the phase velocity of light in a medium is Vph = cjn(w, r, t), where r are coordinates and t is time (of course, n(w, r, t) = n(w) in a homogeneous and stationary medium). Light emission by a charge with velocity v is specified by the ratio vjvph = vnjc. In the vacuum, n = 1 and there is no emission at v = constant (assuming that v < c): it is only possible upon charge acceleration when v = v(t) and, therefore, the acceleration is w = dvjdt =f 0. For a uniform rectilinear motion in a medium, when v = constant and w = 0, the vnj c ratio can change owing to the dependence of non r or (and) t. This represents transition radiation, with n(w, r, t) undergoing alteration at the point of charge location or near it (within the zone of radiation formation).
