SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Circle in a square illusion. The two red circles in the image are the same size. However, the one inside a blue square looks larger than the one enclosing a blue square. This is similar to the Delboeuf illusion and is a demonstration of assimilation - the effect whereby the perceived difference between neighbouring objects is reduced. So the brain treats each circle as being closer in size to its square than it actually is, leading to the apparent discrepancy.
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