  We use the identification between null singularities of correlators in the
bulk with time singularities in the boundary correlators to study the analytic
structure of time-dependent thermal Green functions using the eikonal
approximation for classical solutions in the AdS black hole background. We show
that the location of singularities in complex time can be understood in terms
of null rays bouncing on the boundaries and singularities of the eternal black
hole, giving the picture of a `reflecting cavity'. We can then extract the
general analytic expression for the asymptotic values of the frequencies of
quasinormal modes in large AdS black holes.
