  Gravitational waves act like lenses for the light propagating through them.
This phenomenon is described using the vector formalism employed for ordinary
gravitational lenses, which was proved to be applicable also to a
non-stationary spacetime, with the appropriate modifications. In order to have
multiple imaging, an approximate condition analogous to that for ordinary
gravitational lenses must be satisfied. Certain astrophysical sources of
gravitational waves satisfy this condition, while the gravitational wave
background, on average, does not. Multiple imaging by gravitational waves is,
in principle, possible, but the probability of observing such a phenomenon is
extremely low.
