Muller-Lyer Arrow Illusion
The Müller-Lyer illusion, named after famed German sociologist Franz Carl Muller-Lyer, is a visual phenomenon that plays with our perception of line length and spatial relationships. It consists of two lines of equal length, each capped with arrowheads pointing in opposite directions. The line with outward-pointing arrowheads appears longer than the line with inward-pointing arrowheads. However, when the lines pass each other in the animation, it is clear they are the same length.
It can be attributed to a phenomenon known as "perceptual misinterpretation of angles." Our brains tend to perceive the converging arrowheads as cues for depth, which leads to the misjudgment of the line lengths. The arrows imply that one line is situated closer to us, while the other is farther away, creating a misalignment in our depth perception.
To see this animation, pick the colors you wish for the top and bottom segments respectively and then press animate. You will see the lines switch back and forth, passing on top of each other, proving equal length. Also note that the line that seemed longer after the animation will now be the shorter one!
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