I’ve posted on this before—it’s the most striking optical illusion I’ve seen—but this video is a far better demonstration than the graphics I used before. It’s called the “checker shadow illusion” and was invented by Edward Adelson at MIT. The link in the first sentence explains it: it’s based on how our visual system compensates for shadows, making the checker in shade look lighter than it really is.
I can already hear the theologians crowing (as they often do) about how our senses don’t really perceive an external reality—that our idea that we can trust the world through our senses, as in science, is simply another form of faith. (Forgive me, for I just read this very argument yesterday.)
h/t: Matthew Cobb