Detecting gravito-magnetism with rotation of polarization by a gravitational lens

We discuss the effects of an isolated gravitational lens on the rotation of the plane of polarization of linearly polarized light rays, the so called gravitational Faraday rotation, in metric theories of gravity. By applying the thin lens approximation, we derive simple expressions for extended, rotating astrophysical systems deflecting electromagnetic radiation from background sources. Higher order corrections and the case of a Kerr black hole are also considered to show how the rotation of polarization is a purely gravito-magnetic effect. Prospects of a future detection of a gravitational Faraday rotation are examined.