The Müller-Lyer Illusion is a classic optical illusion that has been studied for over a century. It consists of two lines of equal length, one with inward-pointing arrows at the end and the other with outward-pointing arrows. Despite being the same length, the line
From Wikipedia
The Müller-Lyer illusion is an optical illusion consisting of three stylized arrows. When viewers are asked to place a mark on the figure at the midpoint, they tend to place it more towards the "tail" end. The illusion was devised by Franz Carl Müller-Lyer (1857–1916), a German sociologist, in 1889.