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Muller-Lyer illusion. The two lines with arrows at each end appear to be different lengths on the left. In fact they are the same length. There are many explanations for this, but it is thought likely that it exploits our brain's perception of perspective. Viewing the lines as existing in 3 dimensions and assuming that the arrows are emerging perpendicularly to the lines, we get the impression that the line on the right hand side is closer than the one on the left. As the line at the left is perceived to be further away, it is assumed by the brain to be longer.
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