Mimicking the description of spinning particles in General Relativity, the Fermat Principle is extended to spinning photons. Linearization of the resulting Papapetrou-Souriau type equations yields the semiclassical model used recently to derive the "Optical Hall Effect" for polarized light (alias the "Optical Magnus Effect").PACS numbers: 42.25. Bs, 03.65.Sq, 03.65.Vf, Light is an electromagnetic wave, whose propagation is described by Maxwell's theory. It can also be viewed, however, as a particle (a "photon"). Here we adhere to the second approach: we describe light by a bona fide mechanical model in that we use a Lagrangian.In traditional geometrical optics the spin degree of freedom is neglected, and the light rays obey the Fermat Principle [1]. In the intermediate model advocated by Landau and Lifchitz [2], the photon is polarized, but the polarization is simply carried along by the light rays, and has no influence on the trajectory of light. Recent approaches [3, 4] go one step further : the feedback from the polarization deviates the trajectory from that given by the Fermat Principle. A dramatic consequence is that, for polarized light, the Snel(-Descartes) law of refraction requires correction : the plane of the refracted (or reflected) ray is shifted perpendicularly to that of the incident ray [3]. This "Hall Effect for light" is a manifestation of the Magnus-type interaction between the refractive medium and the photon's polarization [4]. It can be derived in a semiclassical framework, which also includes a Berry-type term [5,6,7].In this Rapid Communication, we argue that the deviation of polarized light from the trajectory predicted by ordinary geometrical optics is indeed analogous to the deviation of a spinning particle from geodesic motion in General Relativity. The resulting equations are reminiscent of those of Papapetrou and Souriau [8].In detail, the Fermat Principle of geometrical optics * UMR 6207 du CNRS associée aux Universités d'Aix-Marseille I et II et Université du Sud Toulon-Var; Laboratoire affiliéà la FRUMAM-FR2291.; Electronic address: duval@cpt.univ-mrs.fr † Electronic address: e-mail:zalanh@ludens.elte.hu ‡ Electronic address: horvathy@lmpt.univ-tours.fr says that light in an isotropic medium of refractive index n = n(r) propagates along curves that minimize the optical length. Light rays are hence geodesics of the "optical" metric g ij = n 2 (r)δ ij of 3-space. To extend this theory to spin we consider the bundle of positively oriented orthonormal frames over a 3-manifold endowed with a Riemannian metric g ij . At each point, such a "Dreibein" is given by three orthogonal vectors U i , V i , W i of unit length that span unit volume. We stress that the [6-dimensional] orthonormal frame bundle we are using here is a mere artifact that allows us to define a variational formalism. Eliminating unphysical degrees of freedom will leave us with 4 independent physical variables.Introducing the covariant exterior derivative associated with the Levi-Civita connection, DU k = dU k + Γ k ij d...