From wikipedia: “The Kanizsa triangle is an optical illusion first described by the Italian psychologist Gaetano Kanizsa in 1955. In the accompanying figure a white equilateral triangle is perceived, but in fact none is drawn. This effect is known as a subjective or illusory contour. Also, the nonexistent white triangle appears to be brighter than the surrounding area, but in fact it has the same brightness as the background.”
