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For this assignment, consider the following illusions: the Muller-Lyer illusion, reversible images, impossible objects, and illusory borders of brightness contrast. What is the illusion and what aspects of perception are involved in creating that illusion?

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The Muller-Lyer illusion is an optical illusion created by Franz Carl Muller-Lyer in 1889. With this illusion, two or more lines are shown, with a fin on the ends of each line. One line has inward-facing fins (like an arrowhead) and the other has outward-facing fins (like the tail of an arrow). The lines with outward-facing fins appear to be longer than the lines with inward-facing fins, though they are actually the same length.

The image attached is a well-known example of the Muller-Lyer illusion. As you can see, each line is the same length, although we perceive them to be different lengths.

There are a few theories to explain how this illusion works:

  1. Psychologist Richard Gregory believes the illusion is due to a misperception regarding size constancy. Size constancy is what allows us to understand how large or small an object is, even at different distances. Gregory suggests that when our minds try to use the size constancy principle on two-dimensional objects, errors sometimes occur.
  2. Some believe depth perception explains the illusion. This explanation states that we perceive the depths of the two shafts differently: we perceive the inward-facing fins as farther away (and therefore smaller) and the outward-facing shafts as closer (and therefore longer).
  3. Another explanation is that we perceive the line with outward-facing fins as longer because the overall figure is longer. Even though the line itself is the same length, the figure with outward-facing fins is longer overall. This causes us to perceive the line as being longer.
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