  This paper is devoted to the complete convergence study of the finite-element
approximation of Maxwell's equations in the case where the magnetic
permeability is constant. Standard linear finite elements for the space
discretization are combined with a well-known explicit finite-difference scheme
for the time discretization. The analysis applies to the particular case where
the dielectric permittivity has a constant value outside a sub-domain, whose
closure does not intersect the boundary of the problem-definition domain.
Optimal convergence results are established in natural norms under reasonable
assumptions, provided a classical CFL condition holds. A numerical validation
of the theoretical results is provided.
