Brewster window
Brewster windows (based on David Brewster ) are used in optical devices that require very little loss, for example in gas lasers . The window surfaces are at Brewster's angle to the optical axis , so that the light polarized parallel to the plane of incidence is not weakened by reflection . However, perpendicularly polarized light is partly reflected as usual. This reduces the quality of the resonator in a laser for one polarization direction, while the other is preferred, which is why the laser will oscillate in this mode .