Abstract: A magnetic mirror for imparting a non-reciprocal phase shift to p-polarized light incident thereon. A magneto-optic garnet layer is formed on a first surface of a transparent substrate having a second opposed substantially parallel surface upon which the light is incident. The light upon passing through the magneto-optic garnet layer is non-reciprocally phase shifted as a consequence of the application of a magnetic field to the garnet layer normal to the plane of incidence. Means are arranged adjacent a surface of the garnet layer for reflecting light passing therethrough back through the layer. As a magneto-optic material, monocrystalline bismuth lutetium iron garnet is particularly advantageous for use as the magneto-optic garnet layer.