A direct proof of Malus' theorem using the symplectic structure of the set of oriented straight lines

We present a direct proof of Malus’ theorem in geometrical Optics founded on the symplectic structure of the set of all oriented straight lines in an Euclidean affine space. Nous pr'esentens une preuve directe du th'eor`eme de Malus de l’optique g'eom'etrique bas'ee sur la structure symplectique de l’ensemble des droites orient'ees d’un espace affine euclidien.