In optics, a caustic or caustic network[1] is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface.[2] The caustic is a curve or surface to which each of the light rays is tangent, defining a boundary of an envelope of rays as a curve of concentrated light.[2] Therefore, in the adjacent image, the caustics can be the patches of light or their bright edges. These shapes often have cusp singularities.