  The principal purpose of this paper is to study the effect of an impulsive
light-like signal on neighbouring test particles. Such a signal can in general
be unambiguously decomposed into a light-like shell of null matter and an
impulsive gravitational wave. Our results are: (a) If there is anisotropic
stress in the light-like shell then test particles initially moving in the
signal front are displaced out of this 2-surface after encountering the signal;
(b) For a light-like shell with no anisotropic stress accompanying a
gravitational wave the effect of the signal on test particles moving in the
signal front is to displace them relative to each other with the usual
distortion due to the gravitational wave diminished by the presence of the
light-like shell. An explicit example for a plane-fronted signal is worked out.
