Another day, another object that shoots rainbows. The things that's beaming a rainbow out into darkness is a Rochon prism. Unlike a regular prism, which just splits light into rainbows, a Rochon prism can actually separate out beams of light.

A Rochon prism is two hunks of clear stuff fastened together. In this case the clear stuff is calcite or quartz, they're fastened together on a diagonal. The calcite is made up of a crystal structure with a certain axis of symmetry — essentially, you could fold the crystal along this axis and the structure of the crystals on either side of the fold matches up. The first hunk of quartz has its axis of symmetry arranged one way. The second one has its axis flipped.

Light moves through the crystal without a problem, but it moves through a little differently depending on whether it's polarized parallel to, or perpendicular to, the axis of symmetry. The light goes through the first side of the calcite crystal, but when it hits the split in the middle and moves into the new crystal, the light that's polarized one way continues straight, while the light that's polarized the other way splits off. This split-off ray is called the "extraordinary ray," and it shoots out from the crystal at an angle. The extraordinary ray is the rainbow we see in the picture above.