Only some humans can see this type of light

Some types of 3d cinema projectors use polarization, including the popular "RealD" technology, so you may have a pair of polarizers lying around. What I've read online says that each eyepiece has a circular polarizer (one clockwise and the other counterclockwise) but the ones I have have at least two polarizers sandwiched together: a circular one in front, and a linear one (the same in both eyes) in the back. You can see that the front is circular by looking through them in the mirror: each eye blocks its *own* reflection, because the mirror reverses the circular polarization. But when you hold them the other way (with the front of the glasses away from the mirror) the light passes through, showing that there is a linear polarizer on that side.