Dynamic Müller-Lyer

What you are looking at

Every horizontal line on this page is exactly the same length. Only the arrowhead fins on their ends move, sweeping between pointing outward and pointing inward. As they do, the lines appear to stretch and contract — the Müller-Lyer illusion, first described by Franz Carl Müller-Lyer in 1889 and animated here after Gianni Sarcone's Dynamic Müller-Lyer, which took third prize at the 2017 Best Illusion of the Year Contest.

Wave staggers the fins so the sweep travels down the stack instead of moving in step. Radial bends the stack into a ring, where the wave becomes a pulse rotating around the circle. The two combine freely. Reveal draws guides through the line ends and pushes the fins into the background, so you can watch every endpoint sit on the same two straight columns while the illusion is still running. The sliders set the number of lines (2 to 80), the animation speed and how long the fins are; on the ring they also set the length of the spokes, and the inner and outer arrowheads can be sized independently, and the spokes lengthened until they just fill the frame — drop the fins to nothing and the illusion disappears entirely. The export button writes a seamless MP4 or GIF loop sized for Instagram, TikTok, YouTube Shorts or Reddit.