How can I tell what the polarization axis is for a linear polarizer?

The axis of a linear polarizer determines the plane of polarization that the polarizer passes. There are two ways of finding the axis of a polarizer. A simple method is to start with a known polarizer with a marked axis. Place both the known and unknown polarizer together and transmit light through them. Rotate the unknown polarizer until no light passes through the pair of polarizers. In this orientation, the unknown polarizer's axis is 90° from the axis of the known polarizer.

If a known polarizer with a marked axis cannot be found, the axis can be found by taking advantage of the Brewster effect. When light reflects at glancing incidence off of a non-metallic surface, the S-polarization is reflected more than the P-polarization. A quick way to do this is to look at the glare off of a tiled floor or another non-metallic surface. Rotate the polarizer until the glare is minimized. In this position, the polarizer is oriented so that the axis is vertical. As an example, sunglasses use polarizers that have the polarization axis vertically oriented.