  Refractive gravitational waves are a generalisation of impulsive waves on a
null hypersurface in which the metric is discontinuous but a weaker continuity
condition for areas holds. A simple example of a plane wave is examined in
detail and two arguments are given that this should be considered a solution of
Einstein's vacuum field equations. The study of these waves is motivated by
quantum gravity, where the refractive plane waves are considered as elementary
quantum fluctuations and the `area geometry' of a null hypersurface plays a
primary role.
