In school I thought that Moire patterns were awesome, and they taught us about how frequencies mix. Years later I would realize that my knowledge of that subject applies to light as well. It is often said that light drives out darkness, but did you know that this only happens for a single light source?
A couple of years ago I noticed the one light in my bathroom was producing a radial pattern of light and dark bands, sixteen altogether. But there was no cover around the light that could produce that pattern. Instead it was a single PL lightbulb, with vertically folded tubes.
It puzzled me for a while, until I thought about the wavelike nature of light: if the waves from two light sources coincide, they add up, but if they are in opposite phase, they cancel out. And that explains why the circular group of light tubes produced a radial pattern of light and dark!