Muller Lyer Illusion: Experiment


The concept of Muller Lyer Illusion was introduced by Franz Carl Muller Lyer (German Psychologist) in the year 1889. It was first published by the physiology journal Archiv für Anatomie und Physiologie, Physiologische Abteilung in 1889.


  1. Three stylized arrows

About Muller Lyer Illusion:

The Muller Lyer Illusion experiment is also called optical illusion due to its characteristic feature which is the identification of the length of two lines. The illusion occurs due to the embezzlement of size constancy scaling. The difference between the sticks is apparent due to the size constancy, in this the lines near our eyes form a bigger image at the retina in comparison to the one placed at far

The arrow on the stitch which is pointing outwards is perceived to be near each other, whereas the arrows pointing towards the line appears to be at a distance which makes the middle line appear to be of a bigger size. 

The lines are placed in parallel to each other with having three different combinations of the arrowheads.

It is based upon the Gestalt principle of convergence and divergence.

This is an important tool for researchers as well as psychologists as it helps in understanding the brain and perceptual processes. 

Piaget in his theory of development (1970) used it to identify the cognitive changes happening in the brain of a child, he defined that the children in the approaching concrete operational stage were able to identify the length of the arrow. 


The subject is shown a line of a particular length with one fixed arrowhead and the other two moveables. One of the moveable arrowheads (pointing outside) is used in fixing a length with the fixed arrowhead. At the back of the apparatus, there is a scale that is only visible to the experimenter and it helps in identifying if the subject has put the moveable arrowhead at the correct distance or is there any difference between the length of the line. 

The subject is asked to move the other moveable arrowhead (pointing inwards) at a length that is equal to the fixed length. The subject can adjust the arrow as per the distance they observe/ assume. 

The length fixed by the experimenter is considered as stimulus and the line to be adjusted is considered as a response. 

The difference between the calculated length and actual length is counted as an error. The errors are calculated on both ends (right and left) being it less or more than the actual length. 

The table for the errors (observation table)

Directions Trials (S-R) (in cm)1      2     3     4      5     6…Mean
Right out
Left out
Right in
Left in 

Uses of Muller Lyer Experiment:

  1. Neurosscientific study
  2. To identify the artistic qualities of a child
  3. Apparel industry 
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