Optics surface within an optical system where incident and conjugate rays appear to refract. This generally curved surface is the locus of intersections of the rectilinearly extended incident and conjugate rays. When only paraxial rays are considered, this surface degenerates to a paraxial principal plane of infinitesimal extent. Paraxial Optics one of two cardinal planes where Gaussian rays appear to refract. Each principal plane lies perpendicular to the paraxial axis and intersects it at a principal point.
Every paraxial system has two principal planes.
For thin-lens and concentric systems, the two principal planes are coincident. Principal points are cardinal points of a paraxial system where the prinicipal plane intersects the paraxial axis. Math one of two mutually perpendicular surface-normal planes that respectively contain the maximum and minimum surface curvatures.