The Jastrow Illusion
Take a look at the image below.
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How does it work?
This is a famous illusion discovered by Joseph Jastrow in 1891. In the illusion two identical curved shapes appear to be different sizes when placed beside each other. The lower shape will always appear longer. Although numerous studies have been conducted into how this illusion occurs, there have been no conclusive explanations for the visual trickery. The most common explanation is that the mind becomes confused by the differing size of the shapes radius, the short side makes the longer side appear shorter. However other explanations include the mind interpreting 2D images as a 3D world, and the mind only attending a small field of vision, using our consciousness to fill in the blanks. Essentially our mind loves to trick us into to seeing what it thinks we should see as opposed to what is actually there.