  We try to go beyond the geometrical optics approximation, by showing that a
massless polarized particle allows a wide class of non minimal interactions
with an arbitrary gravitational field. One specific example of a
curvature-dependent interaction is presented, that results in a
frequency-dependent Faraday effect. Even in a Schwarzschild spacetime, this
leads to the angular dispersion of polarization planes for a linearly-polarized
beam of waves with different frequencies, propagating along the same ray.
