arXiv:hep-th/9602036v2 12 Feb 1996 ON THE SYMPLECTIC STRUCTURES ARISING IN GEOMETRIC OPTICS Jos´ e F. Cari˜ nena † and Javier Nasarre ⋆ † Departamento de F´ ısica Te´ orica, Universidad de Zaragoza, 50009 Zaragoza (Spain) ⋆ Seminario de Matem´ aticas, IFP Miguel Catal´ an, 50009 Zaragoza (Spain) Abstract. Geometric optics is analysed using the techniques of Presymplectic Geometry. We obtain the symplectic structure of the space of light rays in a medium of a non constant refractive index by reduction from a presymplectic structure, and using adapted coordinates, we find Darboux coordinates. The theory is illustrated with some examples and we point out some simple physical applications To appear in Forts. der Phys. Typeset by A M S-T E X 1