A single ray of light enters a semicircular plastic slab and is reflected and refracted by the internal flat surface. Brewster's angle is the angle at which the internally reflected ray is completely polarized in the plane perpendicular to the optical board surface (parallel to the internal surface of the plexiglass slab off which the internal reflection occurs). This occurs when the angle between the refracted and the internally reflected rays is exactly 90 degrees (photograph at left). This can be verified by rotating a polaroid sheet in the light ray coming from the source at the left. When the polarization axis of the polaroid sheet is vertical no internally reflected light is visible (center photograph). Note that the refracted ray is partially polarized at this angle, as seen when the polarizing sheet is rotated so that its axis is vertical (photograph at right).
The lens is used to keep the light ray to a minimum size. Use of the semicircle assures that the incoming and reflected rays will enter and exit normal to the surface and experience no bending.