Rami Arieli: "The Laser Adventure" Chapter 7 Section 5, page 10

 System of mirrors for manipulating the direction of a laser beam

When using a mirror system for redirecting a laser beam, (as seen in figure 7.28), the result is a partial polarization of the beam.

Figure 7.28: System for redirecting a laser beam.

There are some applications in optics where this effect must be taken into consideration.