Caustic of colors in Newton’s prism

Abstract

A single ray of white light, incident on a glass prism, emerges as a family of diverging rays, parameterized by their colors. These originate from a virtual caustic (envelope of colored rays) inside the prism, rather than a focal point. The “caustic of colors” is a singularity different from the familiar colored caustics (incoherent superpositions of monochromatic ray/wave families that themselves possess caustics). Weak dispersion enables analytical approximations: completely describing the virtual caustic, and rendering to simulate its visual colors. The caustic region is very small; observing it with a beam narrow enough to resolve its colors would require a meter-sized prism. Observability depends on the third power of dispersion, so the caustic, though not its colors, might be detectable by extension outside the visual range.