Which arch is bigger? The Jastrow illusion

Blue piece: vertical position (−100 = above, +100 = below):

Animate

Horizontal shift of blue (px):

Rotation of blue:

Inner radius (px):

Thickness (px):

Arc span:

Gap between pieces (px):

The two pieces are identical, yet the lower one looks larger: its long outer edge sits next to the short inner edge of the upper one. Slide the blue piece upward (or press Swap positions) and the illusion reverses — now the orange piece looks larger. At 0% the pieces coincide exactly.

Reference: Jastrow, J. (1892). Studies from the University of Wisconsin: On the judgment of angles and positions of lines. The American Journal of Psychology, 5(2), 214–248. doi:10.2307/1410867

Demo for Lecture: Basic Neuro-cognitive Psychology.
Created by Z. Shi, msense Lab.