A Famous Illusion
All these illusions demonstrate how lines can seem to be distorted by their background.
This illusion is particularly fascinating. The two long lines are equal in length (try measuring them with a ruler or moving the pieces around on the previous page). However, the one with the inward facing arrowheads looks shorter than the one with the outward facing tails.
There are many explanations for how this illusion works. In the twelve years after the Muller-Lyer illusion was first discovered, twelve different theories were put forward to explain it!
The most popular explanation of the Muller-Lyer illusion is that our brain makes mistakes about the relative depths of the two lines. We are used to seeing outside corners of buildings with lines sloping inward away from them. In these situations, the brain knows that the line running down the outside corner is the closest part of the image to us. The brain realises that this line is really shorter than it appears when compared to the rest of the building.
We are also used to seeing the inside corners of rooms with the lines of the roof and floor sloping outward away from them. In these situations, the brain knows that the corner is the furthest part of the image from us. The brain realises that this line is really longer than it appears when compared to the rest of the room.
When the brain compares lines from these two situations to each other, it reduces the size of the line with the inward sloping tails (the corner of the building) because it thinks this line is closer to us. It increases the size of the line with the outward sloping tails (the corner of the room) because it thinks this line is further away. This makes the line with the outward facing tails look longer.
You often see this same depth trick used in paintings. By drawing lines that all slope up or down from corners, people get the impression of depth. Almost all paintings use this kind of perspective drawing.