Müller-Lyer Illusion

Background

The Müller-Lyer illusion (1889) is perhaps the most famous geometric illusion. Two lines of equal physical length appear different depending on the direction of the arrowhead-like “fins” at their endpoints:

Outward fins (>—<): the line appears longer than it really is
Inward fins (<—>): the line appears shorter

Explanations

Several theories have been proposed: misapplied constancy scaling (Gregory, 1963) suggests outward fins resemble the inside corner of a room (perceived as farther away, thus scaled up), while inward fins resemble an outside corner. Confusion theory suggests the visual system averages the overall extent of the figure.

Your Task

A standard line with fins is shown on top. Below it, an adjustable comparison line (no fins) is shown. Use the slider or arrow keys to adjust the comparison line until it appears equal in length to the standard line.

References

Müller-Lyer, F.C. (1889). Optische Urteilstäuschungen. Archiv für Anatomie und Physiologie, Physiologische Abteilung, 2, 263–270.
Gregory, R.L. (1963). Distortion of visual space as inappropriate constancy scaling. Nature, 199, 678–680.
Coren, S. & Girgus, J.S. (1978). Seeing is Deceiving: The Psychology of Visual Illusions. Lawrence Erlbaum.

On each trial, you'll see a line with arrow fins (the standard) and a plain comparison line below it.

Use the slider or ← → arrow keys to adjust the comparison line until it looks equal in length to the standard.

Press Enter or click Confirm when satisfied.

There are 18 trials (3 fin types × 6 repetitions). Take your time on each trial.

Müller-Lyer Illusion

Your Results