The Unexpected Optical Illusion Hidden in a Stack of Chairs

When you’re a scientist who studies vision, everything you see is inspiration. Such is the case for Nicholas Scott-Samuel, a researcher at the University of Bristol studying visual perception, whose latest work was inspired by a stack of chairs in an office. As BPS Research Digest reports, his new article in i-PERCEPTION explores an impossible-seeming optical illusion. It's formed by stacks of a certain type of chair with legs that make a square on each side of the chair (meaning the bottoms of the legs are connected by a bar).

When you look at it head-on, the stack of squares gives you an illusion of great depth, as if the bottom bars of the chair legs stick out rather than stack vertically, and you may not be able to tell which of the legs is on top and which is on the bottom—especially if you’re looking at the drawing of the illusion, rather than at the photo of the chairs.

To explore why a stack of chairs might look so warped, Scott-Samuel and a few other researchers recruited 40 people on Facebook to ask them whether they found the image visually confusing. They did, but only if the stack contained a certain number of chairs—at nine chairs the illusion held, but any number less than four looked totally normal.

The researchers liken the effect to the Penrose triangle, an impossible shape that’s similar to a Möbius strip. But unlike the Penrose triangle, this one is actually possible, as long as you look at it from the correct angle. As the paper states: “Nick Scott-Samuel (in whose office the illusion was observed) reports that it obtains in real life as well as in images, even when sober.”

The paper is extra-short—the author’s bios are longer than the actual contents of the article—demonstrating that science doesn’t have to be a supremely arduous, heavy endeavor. At least not any heavier than a stack of chairs.

Love optical illusions? Try your eye at this one.