  A world sheet in anti-de Sitter space is a timelike submanifold consisting of
a one-parameter family of spacelike submanifolds. We consider the family of
lightlike hypersurfaces along spacelike submanifolds in the world sheet. The
locus of the singularities of lightlike hypersurfaces along spacelike
submanifolds forms the caustic of the world sheet. This notion is originally
introduced by Bousso and Randall in theoretical physics. In this paper we give
a mathematical framework for the caustics of world sheets as an application of
the theory of graph-like Legendrian unfoldings.
