  We present a method developed to deal with electromagnetic wave propagation
inside a material medium that reacts, in general, non-linearly to the field
strength. We work in the context of Maxwell' s theory in the low frequency
limit and obtain a geometrical representation of light paths for each case
presented. The isotropic case and artificial birefringence caused by an
external electric field are analyzed as an application of the formalism and the
effective geometry associated to the wave propagation is exhibited.
