Parallel rays reflecting off a curved mirror produce a caustic — the envelope of the reflected ray family. The bright fold is where ray density diverges. Classic caustics include the cardioid and nephroid.
A caustic is the envelope of a 1-parameter family of lines. For a circular mirror, parallel rays produce a nephroid. For a parabolic mirror, all rays meet at the focal point (perfect focus = degenerate caustic). The catastrophe theory classifies caustic singularities: fold (A₂), cusp (A₃), swallowtail (A₄).