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, and square B is a light tile inside it. A looks clearly darker than B, but they're exactly the same shade of grey. If you press "Show the comparison bridge" in the demo, it draws a solid bar of that grey between the two squares, and as soon as it's there the difference disappears.

Even once you know this, you'll probably find you can't make A and B look the same without the bridge. Take it away again and they split straight back apart.

The brain reports surfaces, not light

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

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

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

Why the bridge breaks it

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

Adelson's demonstration

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