Outlines of the same size fit both squares.
Lighter or darker?
Irradiation illusion
Which square is bigger, the white one or the black one?
They are the same size, though the white one looks bigger.
Why? Light spreads a little in your eye and on your retina, so bright areas bleed into dark ones. Galileo noticed it makes bright planets look bigger than they are.
Picture: Guam, Sopoforic, CC BY-SA 3.0 · Wikipedia(opens in a new tab)