Cornsweet illusion

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The Cornsweet illusion, also known as the Craik–O'Brien–Cornsweet illusion or the Craik–Cornsweet illusion, is an optical illusion that was described in detail by Tom Cornsweet in the late 1960s. Kenneth Craik and Vivian O'Brien had made earlier observations in a similar vein. The original version of the illusion involved a rapidly spinning black-and-white disk, painted in a way that would create the appearance of a gradient effect when in motion. An equivalent static version of illusion is composed of a gray rectangle where the left half fades to a lighter shade as it approaches a vertical center line, and the right half fades to a darker gray approaching the same line. As a result, the whole left half of the rectangle appears lighter than the right half, but in fact the brightness of both areas is exactly the same. This can be seen by blacking out the region containing the edge. This phenomenon is similar to the phenomenon of simultaneous contrast and Mach bands, but differs from it in...

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