Two strips of one gray join A and B.
Lighter or darker?
Checker shadow illusion
Which square is darker, A or B?
Neither: they are exactly the same gray.
Why? Your brain knows that things in shadow get darker, so it makes B look lighter to show you what the square “really” is. It’s helpful in real life, and it fools you here.
Picture: Original: Edward H. Adelson , vectorized by Pbroks13, CC BY-SA 4.0 · Wikipedia(opens in a new tab)