Length of Line Illusion
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Can your eyes deceive you? Adjust the length of the upper red line until you think its length is equal to that of the lower red line. Then check your selection by rotating the top red line to the lower one.
Contributed by: Sándor Kabai (March 2011)
Open content licensed under CC BY-NC-SA
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"Length of Line Illusion"
http://demonstrations.wolfram.com/LengthOfLineIllusion/
Wolfram Demonstrations Project
Published: March 7 2011