This neat optical illusion will have you doubting your eyes for real. These two vertical shapes are actually the same shade. No, really. Place your finger in the middle of the two and you’ll see the different colors. Woah. This is what’s called the Cornsweet Illusion, after ‘experimental psychologist Tom Cornsweet.’ And here’s how it works.
When we look at something, ‘we perceive its color and shading relative to other things in the area and how we perceive it’s lit. In the picture here, we perceive the scene as three-dimensional, with the light source to the upper left.’ Our good ‘ol trusty brain then sees the upper part as lit, while the bottom shape is shadowed. Add the contrast shading between them and our brains interpret the image as two completely different colors.