caustic:(in optics) The curve or surface formed by the reflection of parallel rays of light in a large-aperture concave mirror. The apex of the caustic lies at the principal focus of the mirror. Such a curve can sometimes be seen on the surface of the liquid in a cup as a result of reflection by the curved walls of the cup. A similar curve is formed by a convex lens with spherical surfaces refracting parallel rays of light.
Quotation from: A Dictionary of Physics. Ed. John Daintith. Oxford University Press, 2009. Oxford Reference Online. Oxford University Press.
