Top: the brightness measured across the picture, flat across each stripe. Bottom: how bright it looks, with bands at the edges that aren’t there.
Lighter or darker?
Chevreul illusion
Is each stripe one even gray?
Yes, though each looks lighter next to a darker stripe and darker next to a lighter one.
Why? Cells in your eye that see a bright spot turn down their neighbors. That makes edges stand out, which is useful, but adds bands that aren’t there. Michel Chevreul noticed it in the 1830s, working out colors for a carpet factory.
Picture: Wikimedia Commons, Public domain · Wikipedia(opens in a new tab)