Müller-Lyer Illusion

Background:

Franz Carl Müller-Lyer created a classic illusion in 1889:

Many people perceive the length of the bottom horizontal line as longer than the upper horizontal line even though the two horizontal lines are the same length.

The horizontal lines are called the shafts while the chevrons (< and >) are called the wings.

This demonstration presents five variants of the Müller-Lyer Illusion

1. The classic illusion:
   
2. The illusion without the shaft (the horizontal line) in which you are to judge the distance between the wings (< and >):
   
3. The illusion where the wings have been replaced with circles:
   
4. The illusion with half wings -- either the part of each wing above or below the shaft is missing. This is sometimes called "half fins":
   
5. The illusion with the tips of the inward slanting wings touching. This is sometimes called "intertip":
   

For each variant of the illusion, you will adjust the length of one of the shafts until the two shafts appear to be equally long. After doing this several times, you will be asked to create an explanation as to why one of the variants produced a weaker illusion than the classic version of the illusion. Then you will read a short description of some theories of why the illusion occurs. Finally you will be asked to compare your explanation to one of the theories.

The Study:

Try the variations of the illusion below. Drag the blue pointer that is on the bottom line to adjust the length of one of the shafts until the two shafts appear to be the same length. When you believe the lengths of the shafts are the same, click the "Show values" button. The length of each shaft will be displayed and the absolute value of the difference between the shaft lengths will be added to appropriate cell in the table at the bottom. After two seconds another variant of the illusion will appear. Repeat until you have collected 25 data points.

After completing the 25 judgments, look at the bottom row of the table which shows the mean absolute error for each of the five variants of the illusion. Based on your data, try to create an explanation for why the illusion was weaker for one of the variants than it was for the classic illusion. Then, read the explanations in the link that will appear below the data table once you have completed the study.