I’ve posted before on how liquid-crystal display (LCD) monitors emit polarized light (and can be birefringent), and some of the fun you can have with this, and everyone is probably familiar with other uses as well.

LCD’s don’t emit light by themselves; they rely on backlighting or on reflecting ambient light, which passes through a polarizer behind the display. In this short video we can see what happens with a calculator display when you take the top polarizer off:

We see nothing at all. The effect of the display is not visible to us, because we are not (very) sensitive to polarized light. When we put the polarizing screen in place, then we can see what’s happening — the display has a “zero” energized, which has a different polarization than the rest of the display and blocks the light that has passed through the display and been reflected and polarized. When we rotate the screen, the light is blocked from the rest of the display, and the light from the zero passes through. At an angle, light of each polarization makes it through, so you can’t see the digit at about 45º.