For conformally stationary spacetimes it is possible to formulate the geometric optics approximation of light propagation purely in terms of the geometry of the three-dimensional manifold M consisting of the integral curves of the conformal Killing vector field. In this framework they derive the transport law for the polarization plane in M. If the conformal Killing field has a non-vanishing twist, the polarization plane undergoes a rotation similar to the familiar Faraday rotation in a magnetic field. This so-called gravitational Faraday effect is of particular interest in the gravitational field of rotating matter where is is coupled to the angular momentum of this matter. Therefore, they illustrate their general results with three rotating matter examples, namely (i) an Ozsvath-Schucking spacetime; (ii) the gravitational field outside a rotating body and (iii) a Kerr black hole embedded into a Robertson-Walker cosmos. Thereby they roughly estimate the observability of the gravitational Faraday effect in solar system experiments and in connection with the gravitational lens effect.