  A method to realize artificial magnetic fields for light waves trapped in
passive optical cavities with anamorphic optical elements is theoretically
proposed. In particular, when a homogeneous magnetic field is realized, a
highly-degenerate Landau level structure for the frequency spectrum of the
transverse resonator modes is obtained, corresponding to a cyclotron motion of
the optical cavity field. This can be probed by transient excitation of the
passive optical resonator.
