The Three-Polarizer Paradox is a popular light experiment originally presented by theoretical physicist Paul Dirac in his 1930 book The Principles of Quantum Mechanics.

It is often used to illustrate the strange behavior of quantum mechanics, but it can be explained with classical mechanics as well.

The experiment, described on the reverse of this card, involves 3 identical, linear polarizing filters. Think of these filters as grates with tiny opaque bars crossing it in one direction, as in the image above. The light that passes through the grate is “linearly polarized”, meaning it is only oscillating in that linear plane.

Polarizing filters are great for photography since the effect cannot be created digitally.