Design Nomenclature / Light & Optics
Caustic
Also called caustics, focused light
A concentration of light formed where specular reflection or refraction folds a family of rays onto a diffuse surface — the bright cusp in a coffee cup, the shifting net on a pool floor. The brightness there is not a highlight but genuine energy focused by curvature.
Not to be confused with
- Specular highlightThe reflected image of the source itself in a glossy surface — sharp and small on a polished one, spread and dim on a rough one, since roughness scatters the same energy across a wider lobe. Its position moves with the viewer, which is what separates it from shading.
- Total internal reflectionBeyond a critical angle, light travelling from a denser medium toward a less dense one stops refracting out and reflects entirely back inside. It is why the underside of a water surface reads as a mirror, why fibre optics work, and why a cut gem returns light instead of leaking it.
Origin
Greek kaustikos, burning — from the burning-glass. The mathematical caustic is the envelope of the focused family of rays.