  Plane-symmetric gravitational waves are considered as gravitational lenses.
Numbers of images, frequency shifts, mutual angles, and image distortion
parameters are computed exactly in essentially all non-singular plane wave
spacetimes. For a fixed observation event in a particular plane wave spacetime,
the number of images is found to be the same for almost every source. This
number can be any positive integer, including infinity. Wavepackets of finite
width are discussed in detail as well as waves which maintain a constant
amplitude for all time. Short wavepackets are found to generically produce up
to two images of each source which appear (separately) only some time after the
wave has passed. They are initially infinitely bright, infinitely blueshifted
images of the infinitely distant past. Later, these images become dim and
acquire a rapidly-increasing redshift. For sufficiently weak wavepackets, one
such "flash" almost always exists. The appearance of a second flash requires
that the Ricci tensor inside the wave exceed a certain threshold. This might
occur if a gravitational plane wave is sourced by, e.g., a sufficiently strong
electromagnetic plane wave.
