Optical caustics in the near field from liquid drops

Abstract

When light passes through a small irregular droplet of water on a glass surface the envelopes of the refracted rays form a system of caustic surfaces. The caustics have been examined experimentally, and interpreted theoretically with the aid of Thom’s theorem. The main feature of interest is a unique plane of focus which contains typically several tens of elliptic umbilic catastrophes. These unfold and interact by beak-to-beak events and swallowtail catastrophes to give the many-cusped figures observed close to the drop and in the far field. The primary generic events produced by an irregular drop are analysed by considering them as the unfoldings of a symmetrical case having four control parameters and containing two elliptic umbilics and two butterfly catastrophes.