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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!

Use your mouse to adjust the bottom rectangle so that it matches the top rectangle in length. Now click Show/Measure. How close are you? Click Reset to clear.

 

Credits: Proposed by Mario Ponzo in 1913. Also called the Railway Lines Illusion.
Java Applet: 1997 Tony Azevedo

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