Polarised Light
Light is an electromagnetic wave that travels through space. If you were to consider a transverse wave, from side on, most look like a sine curve, which travel along and vibrate up and down, quite like waves in the ocean. However, the electromagnetic radiation produced by the sun vibrates along many planes, more simply with a vertical and horizontal component. The vertical are the up and down waves, where as the horizontal are left and right waves. This is unpolarised light.
To polarise light is to remove either the horizontal or vertical component of the wave, this is most easily done by filtering. If unpolarised light was shone through a vertical slit, this would eliminate any horizontal component. If that polarised light produced was directed at a horizontal slit, it wouldn’t produce any light due to having blocked the vertical component. The way polarised sunglasses work is the same, they have very thin, vertical slits which block out the horizontal vibration that causes glare.
As for this neat picture I found, it was done similarly. Ice was placed between two polarisation filters, and when the polarised light reflected from the ice, two rays were produced at right angles, one slower from the other. These rays interfered with each other, making all the pretty colours.
