The velocity of the electromagnetic radiation in a perfect dielectric, containing no charges and no conduction currents, is explored and determined on making use of the Lorentz transformations. Besides the idealised blackbody radiation, whose vacuum propagation velocity is the universal constant
c, being this value independent of the observer, there is another behaviour of electromagnetic radiation, we call
inertial radiation, which is characterized by an electromagnetic inertial density
, and therefore, it happens to be described by a time-like Poynting four-vector field
which propagates with velocity
.
is found to be a relativistic invariant expressible in terms of the relativistic invariants of the electromagnetic field. It is shown that there is a rest frame, where the Poynting vector is equal to zero. Both phase and group velocities of the electromagnetic radiation are evaluated. The wave and eikonal equations for the dynamics of the radiation field are formulated.