  We investigate motion of test particles in exact spacetimes with an expanding
impulsive gravitational wave which propagates in Minkowski, de Sitter or
anti-de Sitter universe. Using the continuous form of these metrics we derive
explicit junction conditions and simple refraction formulae for null, timelike
and spacelike geodesics crossing a general impulse of this type. In particular,
we present a detailed geometrical description of the motion of test particles
in a special class of axially symmetric spacetimes in which the impulse is
generated by a snapped cosmic string.
