Optical Illusions

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One type of motion illusion is a type of optical illusion in which a static image appears to be moving due to the cognitive effects of interacting color contrasts and shape position. To properly view this effect, click the image above to see the full sized version.

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The cafe wall illusion is an optical illusion, first described by Doctor Richard Gregory. He observed this curious effect in the tiles of the wall of a cafe at the bottom of St Michael's Hill, Bristol. This optical illusion makes the parallel straight horizontal lines appear to be bent. To construct the illusion, alternating light and dark bricks are laid in staggered rows. It is essential for the illusion that each brick is surrounded by a layer of mortar (the grey in the image). This should ideally be of a color in between the dark and light color of the bricks.

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The Ebbinghaus illusion is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size are placed near to each other and one is surrounded by large circles while the other is surrounded by small circles; the first central circle then appears smaller than the second central circle.

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The illusion is also known as the false spiral, or by its original name, the twisted cord illusion. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.

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The impossible cube or irrational cube is an impossible object that draws upon the ambiguity present in a Necker cube illustration. An impossible cube is usually rendered as a Necker cube in which the edges are apparently solid beams. This apparent solidity gives the impossible cube greater visual ambiguity than the Necker cube, which is less likely to be perceived as an impossible object. The illusion plays on the human eye's interpretation of two-dimensional pictures as three-dimensional objects.