One application of non-reciprocal propagation is to completely block any back-reflection; If the Faraday medium is of such thickness as to cause a 45 degree rotation on the way in, the back-reflected beam will undergo an additional 45 degree rotation in the same sense, resulting in the beam being polarized at 90 degrees, i.e., perpendicular to that of the incident beam, and allowing it to be cleanly blocked by the light source's polarizer.