  We show that the Nariai-Bertotti Petrov type D, homogeneous solution of
Einstein's vacuum field equations with a cosmological constant describes the
space-time in the interaction region following the head-on collision of two
homogeneous, plane gravitational shock waves each initially traveling in a
vacuum containing no cosmological constant. A shock wave in this context has a
step function profile in contrast to an impulsive wave which has a delta
function profile. Following the collision two light-like signals, each composed
of a plane, homogeneous light-like shell of matter and a plane, homogeneous
impulsive gravitational wave, travel away from each other and a cosmological
constant is generated in the interaction region. Furthermore a plane,
light-like signal consisting of an electromagnetic shock wave accompanying a
gravitational shock wave is described with the help of two real parameters, one
for each wave. The head-on collision of two such light-like signals is examined
and we show that if a simple algebraic relation is satisfied between the two
pairs of parameters associated with each incoming light-like signal then the
space-time in the interaction region following the collision is a Bertotti
space-time which is a homogeneous solution of the vacuum Einstein-Maxwell field
equations with a cosmological constant.
