A visual illusion in which a semicircle appears more curved than a shorter circular arc with the same radius of curvature (see illustration). The illusion appears to have been discovered by the English physicist Samuel Tolansky (1907–73), who discussed it in his book Optical Illusions (1964), together with a variation that he called the ‘lens illusion’ involving pairs of curves resembling lenses, commenting that ‘I have noticed that there exists a most curious illusory difficulty when we attempt by eye to assess the radius of curvature’ (p. 51) and that ‘it is strange that these [two] illusions have escaped notice before’ (p. 54). Also called Tolansky's curvature illusion. [From Latin curvatura bending, from curvare to bend, + -ure indicating an action or process, from Latin -ura]
Curvature illusion. The two arcs have the same radius of curvature and are parallel.