The upper horizontal line segment in the above figure appears to be longer than the lower line segment despite the fact that both are the same length.
Ponzo's Illusion
See also
Illusion, Müller-Lyer Illusion, Poggendorff Illusion, Vertical-Horizontal IllusionExplore with Wolfram|Alpha
References
Fineman, M. The Nature of Visual Illusion. New York: Dover, p. 153, 1996.Referenced on Wolfram|Alpha
Ponzo's IllusionCite this as:
Weisstein, Eric W. "Ponzo's Illusion." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PonzosIllusion.html