A magnetic twist that steers light

Put on a pair of polarized sunglasses, look at your phone, and slowly rotate the phone. On many phones, the screen goes dark at a particular angle. That happens because the screen emits polarized light: its electric field oscillates in a particular direction. The lenses let through light polarized along one direction and absorb light polarized perpendicular to it. When you rotate the phone so that the screen’s polarization lines up with the direction the lenses absorb, the light is blocked and the screen appears dark.