Zollner Illusion

The Zollner illusion is an optical phenomenon named after its discoverer, the German astrophysicist Johann Karl Friedrich Zollner, who communicated the discovery to the physicist Johann Christian Poggendorff in 1860; Poggendorff, examining Zollner's original drawing, in turn identified the separate, related Poggendorff illusion within it. The illusion consists of a series of parallel diagonal lines crossed by short, repeating lines whose direction alternates between horizontal and vertical; this pattern creates the false impression that the long diagonal lines are diverging from one another, because the angled short lines create a sense of depth suggesting one end of each long line is closer to the viewer than the other, a mechanism similar to the one behind the Wundt illusion. The Zollner illusion belongs to a family of related illusions, including the Hering illusion, the Poggendorff illusion, the Muller-Lyer illusion and the Cafe wall illusion, all of which show how background elements can distort the perceived path of a line.