The larger an arc is, the smaller its radius appears. For example, the three arcs illustrated above belong circles of the same radius.
Three-Arc Illusion
This entry contributed by Margherita Barile
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References
Carraher, R. G. and Thurston, J. B. Optical Illusions and the Visual Arts. New York: Reinhold, 1966.Referenced on Wolfram|Alpha
Three-Arc IllusionCite this as:
Barile, Margherita. "Three-Arc Illusion." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/Three-ArcIllusion.html