A magnetic twist that steers light

Put on a pair of polarized sunglasses, look at your phone, and slowly turn it. On most phones, at one angle the screen goes dark. The screen sends out polarized light: its electric field oscillates along a single direction. The lenses absorb light polarized along one direction and let the perpendicular one through, so when the screen’s direction matches the one the lenses absorb, nothing gets through. That direction is fixed once and for all when the lenses are made.