Check out Michael Bach’s amazing and expansive collection of optical illusions. Some of them will undoubtedly be familiar to you from your youth, and you’ve probably also seen a couple of the more sophisticated ones on computers more recently. But they’re all fascinating and worth perusing for an hour or two. He introduces his site thusly:
“Optical illusion” sounds pejorative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as bringing out particular good adaptations of our visual system to standard viewing situations. These adaptations are hard-wired in our brains, and thus under some artificial manipulations can cause inappropriate interpretations of the visual scene. As Purkinje put it: “Illusions of the senses tell us the truth about perception.”
You can click on his first one and then keep clicking “Next” to browse through all of his illusions, or you can keep returning to his list to pick and choose which ones to visit. I recommend the former method. If you’re short on time, or lazy, or impatient, I’ll tell you my three favorite: the lilac chaser, the spiral aftereffect, and my favorite optical illusion of all time, Ted Adelson’s checker-shadow illusion.