Something you put in your drinks has a special property that, along with special polarization, causes it to erupt into rainbows. Find out what it is, and what makes it so colorful.
This is a thin section of an ordinary ice cube. Seen through a pair of polarized filters, it’s colored because ice has the property of birefringence. This means that the speed at which light travels through ice, or any birefringent substance, depends on both the polarization of the light and the angle at which light hits the ice crystal. In this case, it’s ice crystals—there are several differently oriented lattices in the section. Beams of light, which have been previously polarized by passing through a filter, will be split up into different waves, and those waves will travel through the ice at different speeds, moving out of phase with each other.
Meanwhile, a second polarizing filter will filter out some of the light that comes out of the crystal of ice. The resulting waves interfere with each other, some peaks heightening, and some canceling each other out. The interference gives us the same chaotic rainbow that we get from soap bubbles.
Plastics are also often birefringent. Breaks or stresses are what cause the change in color, so cross-polarization photography is a common way for manufacturers to know if they have any weak spots, or places that undergo high stress in plastic forks, knives, or other tools.
