Müller–Lyer Illusion, explained.
The Müller–Lyer illusion is a difference in apparent line length associated with different fin arrangements at the endpoints.
Why it happens
A fixed 240-unit upper shaft is compared with a lower shaft adjustable from 180 to 300. Hiding fins preserves both physical shaft lengths, revealing the selected difference.
The Müller–Lyer illusion is a difference in apparent line length associated with different fin arrangements at the endpoints.
Read the result
The comparison concerns the horizontal shafts. Fins do not count toward the reported shaft length.
A worked example
A length comparison with fins
Adjust the lower shaft while both sets of opposing endpoint fins remain visible.
Hide the fins and compare the fixed upper length of 240 with your selected lower length.
Restore the fins to inspect context again; the drawing does not impose a particular perceptual error.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
A fixed 240-unit upper shaft is compared with a lower shaft adjustable from 180 to 300. Hiding fins preserves both physical shaft lengths, revealing the selected difference.
Where this idea is useful
A practical use
A diagram’s decorations can affect judgments even when its measured segments stay fixed.
A common misconception
“The longer overall decoration must contain the longer shaft.”
Overall decoration and endpoint-to-endpoint shaft length are different quantities. The reveal isolates the quantity being compared.
What this explanation leaves out
- An authored drawing with visible dimensions cannot establish a universal apparent-length difference.
Is the selected difference an illusion-strength measurement?
It is a record of one adjustment with visible numeric information, not a calibrated or controlled psychophysical estimate.
Which decorative endpoints might change how a reader estimates distance?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.