• Background
• Instructions
• Illustration
• Quiz

## Background

In the Müller-Lyer illusion (see below), we see the left line as longer than the right line, even though both lines are exactly the same length. If you do not believe this assertion, measure the two lines with a ruler. They are the same length. We will not distort illusions in this textbook, but please do not trust us. Measure our illusions objectively, and you will discover how your perception can be tricked. With respect to the Müller-Lyer illusion, it is the smaller lines that split off from the main line that create the illusion that the main line is longer or shorter. An obvious question is, Why would these additional lines affect our perception of the length of the longest line? Most explanations of the Müller-Lyer illusion focus on the relation of size and depth. For example, Gregory (1966) advanced the view that the Müller-Lyer illusion is the result of misapplied size constancy. What this means is that the visual system wants to keep objects of the same size looking the same size, but in the case of the Müller-Lyer illusion, we mistakenly see size differences when the size is actually the same.

The argument works as follows. Consider the left image in the Müller-Lyer illusion. Think about what this might look like in three dimensions. We may see the image as a corner in a wall. The corner is close to us and the little projections at the top and bottom point away from us, as they might if the corner were near us, and the walls led away. In the image on the right, we see the corner as being farther away, and the little projections that mark the corner are coming toward us. Here we see the corner as being at a distance and the walls coming toward us. Because we see the line as being farther away in the right-hand image than we do in the left-hand image, we see the line in the right-hand image as longer. Why? Because it takes up the same space as the left-hand line on our retina, but we perceive it as being more distant. Objects that take up the same amount of space on our retina, but are more distant, are necessarily larger. Hence, we see the line as longer. See the text for how this argument would work in the real world.

Use this activity to try the Müller-Lyer Illusion and change several parameters about the illusion to see how impacts the strength of the illusion.

## Instructions

### Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

### Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Line Length: adjust the length of the right hand line to make it appear the same length as the left hand line.
Match: if you think the two lines look the same length, click here to remove the arrow heads at each end and see just the lines to see if the lines still appear the same length.
Head Angle: change the angle of the arrow heads to be either sharper or more at a right angle from the main line.