  Some strong field effects on test particle motion associated with the
propagation of a plane electromagnetic wave in the exact theory of general
relativity are investigated. Two different profiles of the associated radiation
flux are considered in comparison, corresponding to either constant or
oscillating electric and magnetic fields with respect to a natural family of
observers. These are the most common situations to be experimentally explored,
and have a well known counterpart in the flat spacetime limit. The resulting
line elements are determined by a single metric function, which turns out to be
expressed in terms of standard trigonometric functions in the case of a
constant radiation flux, and in terms of special functions in the case of
oscillating flux, leading to different features of test particle motion. The
world line deviation between both uncharged and charged particles on different
spacetime trajectories due to the combined effect of gravitational and
electromagnetic forces is studied. The interaction of charged particles with
the background radiation field is also discussed through a general relativistic
description of the inverse Compton effect. Motion as well as deviation effects
on particles endowed with spin are studied too. Special situations may occur in
which the direction of the spin vector change during the interaction, leading
to obsevables effects like spin-flip.
