  A potential for propagation of a wave in two dimensions is constructed from a
random superposition of plane waves around all propagation angles.
Surprisingly, despite the lack of periodic structure, sharp Bragg diffraction
of the wave is observed, analogous to a powder diffraction pattern. The
scattering is partially resonant, so Fermi's golden rule does not apply. This
phenomenon would be experimentally observable by sending an atomic beam into a
chaotic cavity populated by a single mode laser.
