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The Müller-Lyer Illusion

Here are two verticle lines of identical length. One has reversed arrowheads while the other's look normal. One of the lines looks longer!

The arrowheads are distorting our perception, and it may be our "built-in" sense of perspective which causes the distortion.

Adjust the blue line on the left by pulling the top, or bottom end. When both lines appear to be the same length, press show to measure. Press reset to equalize line lengths.
    

Credits: Illusion by Franz Müller-Lyer, 1889.
Java Applet: ©1998 Tony Azevedo

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