Adelson's Checker-Shadow Illusion Adelson’s Checkershadow Illusion Adelson’s Checkers Shadow illusion exploits the mechanisms underlying lightness constancy: our capacity to perceive the lightness (or reflectance) of a surface as invariant, even when the intensity of incident light (the illuminance) is changing at a point or is variable across the surface.
It’s impossible for the Impossible Trident to exist because in order for it to exist rules of Euclidean geometry would have to be violated. For example, the trident appears at one end to have merely two prongs, but at the other end to have three, simultaneously.
This is an animation of Shepard’s Tables, an illusion first published by Roger Shepard as Turning the Tables, (see his wonderful book Mind Sights, 1990, pages 48 and 127-8). The left hand lozenge-shaped table top seems to get longer and thinner as it rotates, but it’s an illusion.