Checker Shadow Illusion

Two greys that look nothing alike

A checkerboard lies partly in the shadow of a cylinder. Square A is a dark tile outside the shadow; square B is a light tile inside it. A looks clearly darker than B, but they're exactly the same grey. The bridge button in the demo draws a solid bar of that grey between them, and as soon as it's there, the difference disappears.

Even knowing this, most people can't make A and B look the same without the bridge. Take it away and they split apart again.

The brain reports surfaces, not light

Your visual system isn't a light meter. It doesn't try to tell you how much light is reaching your eye from each point; it tries to tell you what each surface is like, regardless of how it's lit. A white shirt in shadow sends less light to your eye than a grey shirt in sunshine, yet you see the first as white and the second as grey, because the brain discounts the lighting and estimates the material. This is called lightness constancy.

The checker shadow picture triggers that process on purpose. The darker region looks like a shadow, so the brain discounts it and judges the tiles inside it as lighter than the light reaching your eye suggests. The tile outside the shadow gets no such correction. The brain is doing exactly the right thing for a photo of a real shadowed checkerboard; it just happens that this one is a drawing with the numbers rigged.

The pattern of the board helps. A is a dark tile surrounded by light tiles, which makes it look darker still; B is a light tile surrounded by dark ones, which makes it look lighter. That's simultaneous contrast, the same effect in its simplest form: lightness is always judged against what's around it.

Why the bridge breaks it

The bar works because it removes the boundary the brain was using to separate "in shadow" from "out of shadow". Once one continuous strip of a single grey visibly joins A and B, the visual system can't treat its two ends as two different surfaces under two different lights, so the correction that pushed them apart has nothing to act on.

Adelson's demonstration

Edward Adelson, a vision scientist at MIT, published the illusion in 1995 to demonstrate lightness constancy (Adelson, 1995), and it became one of the most widely shared illusions online, partly because it survives being told the answer in advance. His later review explains it as the visual system grouping the image into regions that share the same lighting and judging lightness within each (Adelson, 2000). Together with "the dress", it's one of the clearest public demonstrations that the colours and greys you see are the brain's estimates, not a direct reading of the light.