Diffraction in lips and beak-to-beak caustics

When an initially smooth wavefront is allowed to propagate, the first caustic to appear on a transverse screen is typically a lips. Lips and beak-to-beak caustics are tangential sections through three-dimensional cusp caustics that have curved ribs. They have universal shapes and this paper analyses the diffraction patterns that would be seen on a screen accompanying the caustics. These form two universal one-parameter sets, depending on how much of the cusp diffraction (Pearcey) pattern is intercepted, unlike the canonical catastrophes, which all have universal forms. An experimental diffraction pattern obtained by passing light through a water drop is compared with a computed beak-to-beak pattern.