Same color illusion

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The same color illusion — also known as Adelson's checker shadow illusion, checker shadow illusion and checker shadow — is an optical illusion published by Edward H. Adelson, Professor of Vision Science at MIT in 1995.^[1] The squares A and B on the illusion are the same color (or shade), although they seem to be different.

"When interpreted as a 3-dimensional scene, our visual system immediately estimates a lighting vector and uses this to judge the property of the material."^[2]

The left image below shows what appears to be a black and white checker-board with a green cylinder resting on it that casts a shadow diagonally across the middle of the board. The black and white squares are actually different shades of gray. The image has been constructed so that "white" squares in the shadow, one of which is labeled "B," are actually the exact same gray value as "black" squares outside the shadow, one of which is labeled "A." The two squares A and B appear very different as a result of the illusion. A second version of the same picture includes a rectangular bridge connecting square A and B to show they are the same shade of gray.