The Müller-Lyer illusion can be demonstrated in two ways:
- Standard two-dimensional figures: Two vertical lines of identical length appear different because one line has V-shaped brackets angled toward its center and the other has V-shaped brackets angled away from its center, making the latter line appear longer.
- Three-dimensional application: Two lines of identical length appear different when placed in a drawing of walls; a line bordering a window on a wall that appears farther away is perceived as shorter than a line marking a 90-degree corner of a wall that appears closer.

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The Müller-Lyer illusion is a visual phenomenon where two lines of identical length appear to be different lengths due to V-shaped fins attached to their ends. A line with inward-pointing fins is perceived as shorter than a line with outward-pointing fins. This illusion is a key example in the study of perception, and susceptibility to it has been shown to vary across cultures, with individuals from Western cultures often being more prone to experiencing it.

Müller-Lyer Illusion

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

One explanation for the Müller-Lyer illusion is based on depth perception, suggesting that our brains interpret the 2D figures using 3D spatial cues. The line with outward-pointing fins resembles the outside corner of a building projecting toward the viewer (appearing closer), while the line with inward-pointing fins can be seen as the inside corner of a room receding into the distance (appearing farther away). According to the principle of size constancy, if two objects create retinal images of the same size, the object perceived as being farther away is interpreted as being physically larger. This principle helps explain why the different fin orientations lead to a misperception of line length.

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