Malus-Dupin theorem
Theorem in geometrical optics / From Wikipedia, the free encyclopedia
The Malus-Dupin theorem is a theorem in geometrical optics discovered by Étienne-Louis Malus in 1808[1] and clarified by Charles Dupin in 1822.[2] Hamilton proved it as a simple application of his Hamiltonian optics method.[3][4]
Consider a pencil of light rays in a homogenous medium that is perpendicular to some surface. Pass the pencil of rays through an arbitrary amount of reflections and refractions, then let it emerge in some other homogenous medium. The theorem states that the resulting pencil of light rays is still perpendicular to some other surface.