  The existence of large--scale magnetic fields in the universe has led to the
observation that if gravitational waves propagating in a cosmological
environment encounter even a small magnetic field then electromagnetic
radiation is produced. To study this phenomenon in more detail we take it out
of the cosmological context and at the same time simplify the gravitational
radiation to impulsive waves. Specifically, to illustrate our findings, we
describe the following three physical situations: (1) a cylindrical impulsive
gravitational wave propagating into a universe with a magnetic field, (2) an
axially symmetric impulsive gravitational wave propagating into a universe with
an electric field and (3) a `spherical' impulsive gravitational wave
propagating into a universe with a small magnetic field. In cases (1) and (3)
electromagnetic radiation is produced behind the gravitational wave. In case
(2) no electromagnetic radiation appears after the wave unless a current is
established behind the wave breaking the Maxwell vacuum. In all three cases the
presence of the magnetic or electric fields results in a modification of the
amplitude of the incoming gravitational wave which is explicitly calculated
using the Einstein--Maxwell vacuum field equations.
