Checker Shadow Illusion →

One of my favorite optical illusions is MIT professor Edward H. Adelson's "Checker Shadow Illusion". It's one of those illusions that your brain simply refuses to see.

Look at the image above. Square "A" and square "B" are the exact same shade of gray. See it? Probably not. No, I'm not tricking you, they really are the exact same shade.

The illusion has just two components; a checkerboard and a cylinder (let's say it's a SpaghettiO's can). The can's shadow on the center of the board is the key that makes this illusion work.

When the SpaghettiO's can casts a shadow, the light-colored and dark-colored squares that it covers are darkened equally. When we look at square B, our brain decides that it's a "white" square because its contrast is much lighter than the surrounding squares. When we look at square A, our brain decide that it's a "black" square for a the opposite reason; its contrast is much darker than its paler neighbors. We decide if a square is light or dark based on the contrast between itself and its neighbors, not by comparing the luminescence of the two squares directly. Whether we like it or not, our eyes simply can't resist comparing squares A and B to the ones around it, and that's what messes us up; they really are exactly the same shade of gray.

Youtuber brusspup decided to recreate the famous illusion using a real light source and board, the result looks like some sort of magic or fancy CGI:

If you're still not convinced, take a look at this image that bridges the gap between squares A and B with a gray bar:

If you still refuse to second-guess what your trusty eyes are telling you, print out the first image and cut the two squares out. It's a pretty fun illusion, huh?