  We show exact solutions of Born-Infeld theory for electromagnetic plane waves
propagating in the presence of static background fields. The non-linear
character of Born-Infeld equations generates an interaction between background
and wave that changes the speed of propagation and adds a longitudinal
component to the wave. As a consequence, in a magnetic background the ray
direction differs from the propagation direction --a behavior resembling the
one of a wave in an anisotropic medium--. This feature could open up a way to
experimental tests of Born-Infeld theory.
