Checker Shadow

What is happening

This checkerboard and its diagonal shadow are built here from the general principle of brightness constancy — this is not a copy of the famous cylinder figure. A is the dark square outside the shadow, B is the light square inside it. Compare the brightness of the squares themselves, not the letters printed on them.

Why it looks that way

Your visual system tries to separate an object’s own color from the effect of lighting on it. A light square inside a shadow and a dark square outside one get read as different contexts. So even though they are the same gray in the image, the impression of how bright the underlying object is can end up mismatched.

How to check

Revealing the answer connects the two squares with a band of the same gray. Notice how the boundary between the band and the squares becomes hard to see. You can also hide the surrounding checkerboard, and bring the pattern back to see the difference return — though how strong the effect feels varies by display and by person.

Try a related illusion

For reference: Michael Bach’s explanation. The figure here is an original implementation for this site.

First described

After Edward H. Adelson, 1995

Massachusetts Institute of Technology. Only the general principle is referenced; the figure here is rebuilt from scratch for this site.

See it in the timeline