What do you think of this fact?
The picture looks harmless: a checkerboard, a green cylinder, a shadow. Two squares are labeled A and B. A looks clearly dark, B looks clearly light. They are exactly the same gray.
Same pixels, two worlds
The checker shadow illusion was published in 1995 by Edward H. Adelson, a professor of vision science at MIT. Square A sits in the light, square B in the cylinder’s shadow. Your brain is certain they are two completely different colors.
Michael Bach, who collects optical illusions on his website, puts it plainly: both squares have the same luminance, so the same amount of light comes off each. Cover everything around them so that only the two squares are visible, and the difference vanishes instantly. Take the cover away and it snaps right back.
Your brain is not a light meter
Why does this happen? According to the New World Encyclopedia, your visual system reads the image as a three-dimensional scene. It works out where the light is coming from, notices the shadow and corrects for it. A square that still looks medium gray inside a shadow must, by that logic, be a light square. A square in full light with the same gray must be a dark one.
Local contrast adds to it. B is surrounded by darker squares, which makes it look lighter. A is surrounded by lighter squares, which makes it look darker. And the checkerboard pattern itself tells your brain which squares ought to be dark and which light.
Bach’s conclusion: eyes don’t work like an exposure meter, they use context. So when you ask “which square is brighter?”, your brain actually answers a different question: is this a light or a dark square on this board, whatever the lighting? And it gets that one completely right.
Bonus fact: Shadows usually have soft, fuzzy edges while painted edges are sharp, and your visual system uses that difference to tell shadows apart from real surface color.
Your brain doesn’t show you what is in front of you. It shows you what it thinks is most likely there.