Polarizer

a polarizer of opinions

When the polarizers are aligned in the same direction, they block the same state of polarization, i.e. they both block horizontally polarized light, and both let the vertically polarized light through. The second polarizer therefore has no effect on the light. When the polarizers are aligned perpendicular to one another, one will block the horizontally polarized light and the other will block vertically polarized light — no light at all gets through the pair!