The Muller-Lyer illusion is a size constancy illusion. In this illusion, the red and the blue lines below are the same length, but due to the effects of the arrows the red line appears longer.
The red and blues lines are both 170 pixels long
The illusion has been well studied since it was described by German psychiatrist Franz Carl Muller-Lyer in 1889. However, there is no certain explanation for the apparent difference in the perception of the lengths.
Muller-Lyer attempted to explain the illusion he had discovered as follows: "the judgment not only takes the lines themselves, but also, unintentionally, some part of the space on either side."
One of the better explanations I've read comes in the form of a PDF file from Catherine Q. Howe and Dale Purves of the Center for Cognitive Neuroscience, Duke University.
Below is an amazingly strong illusion that combines both the Ponzo and Muller-Lyer optical illusions.
The two red vertical lines are the same length.
Placing your pointer over the image will aid you in exposing this illusion.
In the tickets image above the perspective of distance lends itself to the Ponzo optical illusion, the angles at the end of each line add to this the Muller-Lyer illusion, combined they create a very effective optical illusion.
Project: Create your own illusion. See how the following variations might effect the relative strength of this illusion.
Does changing the color of the background change the effect?
Does changing the color of the arrows change the effect?
Try changing the color of the equal lines,
is there any combination of colors that is stronger then the others?
Try fat lines. Try skinny lines. Does the thickness of the lines affect this illusion?
Does the distance from which you view the illusion change the illusion?
What can you conclude, if anything, about how we perceive things from this project?