When observing a distant point source of light through a water droplet adhering to a pane of
glass near one's eye or the scattering of light from raindrops, one often sees optical caustics.
In this paper, diffraction integrals are used to investigate these caustics. The caustic shapes
are related to divergences in the stationary phase method for approximating the diffraction
integrals. These divergences correspond to the coalescing of two or more geometrical light
rays in ray optics or the coalescing of two or more regions of stationary phase in wave optics …