Between light and shadows-a brief history of caustics: retrospective.

A heuristic definition of caustics is simply the locus of light-intensity concentration. However, zooming into caustics reveals subtle structures; unraveling them involves employing sophisticated mathematics dealing with singularities. This paradoxical epistemic duality has made caustics a crucial concept permeating the evolution of optics itself since its origins. Moreover, the seemingly capricious caustic shapes found in nature-the rainbow being the most inspiring theme-have stimulated some of the sharpest minds in science throughout the ages. The ever-deepening understanding of the structures of caustics has co-evolved with various branches of mathematics, such as geometry and divergent series. I present a historical review of this profound co-evolution from ancient times to the 80s of the 20th century.