Making plastic into rainbows would nice enough even as a hobby — but it turns out there's an entire field of study behind this light display. It's called photoelasticity, and it helps people check where the lines of tension are on a structure.
When light moves through a material, its angle of movement is generally changed just slightly. The process is analogous to a person on roller skates going from, say, smooth asphalt to slightly rougher concrete. If they hit the concrete straight on, both skates hit at the same time and the person doesn't change their angle of movement. If, however, they hit the concrete so that their right skate hits just before their left, their right skate will slow down while their left one will keep speeding at the same velocity until it, too, hits the concrete. At that point, both skates will be going at a slower speed, and they'll continue on their merry way at a slightly different angle. This will swing them around to the right a bit. The same thing happens when light hits glass, or water, or plastic. It slows down and its angle changes, depending at what angle it hit the new medium.
But some materials seem to have a property called birefringence. These materials don't change the angle of light uniformly, they change it depending on the polarization of the light that hits them. So vertically polarized light might swing to the right quite sharply, while horizontally polarized light will just change direction slightly. Photoelastic materials show birefringence when the material is under stress. The more stress that it is under, the more dramatically birefringent it is.
Polarized light is sent into the material, and, depending on the stress, the two differently refracted waves of light interfere with each other in various ways that allow the viewer to measure the levels of stress on the material. It's useful information... that also happens to be very, very pretty.
Top Image: Nevit Dilmen