Optical illusions generally occur in the way scenes are processed in the brain. Typically they take advantage of the way our brains have evolved to see the world. For an example, look up the Ponzo illusion. This illusion involves two converging lines, with two bars of equal size placed at difference points along those converging lines. In the natural world, we see converging lines when things are getting farther away (think of looking down a long, straight road or railroad track). When something is far away, even if it appears small we know that it might be quite big, because things appear smaller the farther away they are. The Ponzo illusion makes the bar that is further up the converging lines appear farther away. Because we know that things that are farther away are bigger than they appear, the bar that is further up seems bigger than the one further down. Physically, our eyes are the same when experiencing an optical illusion, the illusions just trick our perceptual systems in our brains.