  We focus on the interaction of a plane gravitational wave with
electromagnetic fields and we describe this interaction in the proper detector
frame where, thanks to the introduction of Fermi coordinates, it is possible to
refer to directly measurable quantities. The presence of a gravitational field
can be addressed in terms of an effective electromagnetic medium and, within
this framework, we show that the coupling of pre-existing electromagnetic
fields with the gravitational field of the wave gives rise to new effective
currents. To assess the impact of these effects, we solve Maxwell's equations
for some standard configurations of the electric and magnetic fields.
