The same block without the light and dark edge in the middle: one even gray, top to bottom.
Lighter or darker?
A cube in the sun
The top of the block is dark gray and the bottom white. Is it?
No: away from the edge in the middle, the top and the bottom are exactly the same gray.
Why? Only near the edge between them does the top get darker and the bottom lighter, as in the Cornsweet illusion. Your brain reads the edge as the line between a shaded half and a sunlit one, and paints each whole half to match. Dale Purves and Beau Lotto made the first; this is our drawing.
Picture: Drawn by Science Playground, after Dale Purves and R.… · Wikipedia(opens in a new tab)