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The Ponzo Illusion

Understanding the Ponzo Illusion

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In the Ponzo illusion, two identically-sized lines appear to be different sizes when placed over parallel lines that seem to converge as they recede into the distance.
ponzo illusion

The Ponzo Illusion

Image from Wikimedia Commons

What Do You See?

In the image above illustrating the Ponzo illusion, the two yellow lines are the exact same size. Because they are placed over parallel lines that seem to converge in the distance, the top yellow line actually appears to be longer than the bottom one.

How Does the Ponzo Illusion Work?

The Ponzo illusion was first demonstrated in 1913 by an Italian psychologist named Mario Ponzo. The reason the top horizontal line looks longer is because we interpret the scene using linear perspective. Since the vertical parallel lines seem to grow closer as they move further away, we interpret the top line as being further off in the distance. An object in the distance would need to be longer in order for it to appear the same size as a near object, so the top "far" line is seen as being longer than the bottom "near" line, even though they are the same size.

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