Slid together into one ring, the two halves look the same gray.
Lighter or darker?
Koffka ring
Are the two halves of the ring the same gray?
Yes, though the one on the light background looks darker. “Prove it” slides them together into one ring, and then they look the same.
Why? Apart, your brain compares each half with the background around it. Joined into one ring, it treats the ring as one thing with one color.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)