Caustic surface, (from the Greek kaustikos — burning) in optics, the surface that envelops a family of light rays emitted by a luminous point and passed through an optical system. Put otherwise, the caustic surface is the surface at every point of which two rays diverging from the luminous point at a very small angle intersect after being refracted at the boundaries between the optical media of the system. The caustic surface is clearly visible in a smoky medium, because light energy is concentrated on it. The aberrations of optical systems can be classified by the symmetry properties of the caustic surface. For example, an axisymmetric caustic surface corresponds to spherical aberration, and a caustic surface symmetric about some meridional plane — to coma. In aberration-free optical systems the caustic surface becomes a point — the image of a point source.