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The Ponzo Illusion

Two yellow rectangles of equal size are placed over a set of railway tracks. Our sense of 'rightness' tells us that the top rectangle should appear smaller, being farther away. Yet it appears bigger!

Credits: Proposed by Mario Ponzo in 1913. Also called the Railway Lines Illusion.
Java Applet: 1997 Tony Azevedo
modified for instuctional (changing variables and quantitative measurements), Eugene Switkes, CfAO/COSMOS Program 2001