JETP VOLUEM 36 9 NUMBER 3 SEPTEMBER 1959 THE ROLE OF GROUP VELOCITY OF LIGHT EMITTED IN A REFRACTIVE MEDIUM
THE Vavilov Cerenkov effect has been extensively used recently in research in nuclear physics, particularly the physics of high energy particles. At the present time one observes in the experiments either radiation from individual charged particles that move in a refractive medium or the total incoherent radiation produced by many charges that move independently of each other. However, this immediately raises the question of radiation by a moving system of particles. A system of particles may have a natural frequency of oscillation. Therefore, an examination of the behavior of a radiator of arbitrary natural frequency moving in a medium may be not merely of fundamental but also of practical interest. The purpose of this article is to clarify the role of the group velocity of light in radiation from such a source, moving uniformly in a refractive isotropic medium that is transparent to the radiated light. It is known that the radiation from a moving source of light depends substantially on the ratio of the radiator velocity to the phase velocity of light at the radiated frequency. Thus, a particle producing a time invariant electromagnetic field (electric charge, fixed magnetic dipole, etc. i.e. a particle with a zero natural frequency, is subject to the Vavilov Cerenkov effect if its velocity of motion reaches or exceeds the phase velocity of the light. Here the connection between the direction of the emission of light and the radiated frequency we is determined by the ratio of the velocity of motion v to the phase velocity of the light u c/n (we for this frequency, namely