Sander Illusion

In the Sander Illusion, the diagonal line on the left appears to be considerably longer than the diagonal line on the right, but the lines are the same length.

Sanders illusion
Sander Illusion


Table of Contents


How does the Sander Illusion work?

The Sander illusion works by exploiting the brain’s tendency to interpret the visual information it receives in the context of other visual information.

This illusion is thought to be caused by the visual system’s use of depth cues, such as the size and angle of the lines

The perception of depth and distance are affected by the context of the arrowheads, which fool the visual system into perceiving the lines as if they were in different distances and therefore different sizes.

Another explanation of the illusion is based on the concept of “Gestalt grouping” where the brain organizes different elements in a scene into groups based on certain rules such as proximity, similarity, and continuity. The triangles in the Sander illusion creates a sense of continuation and proximity between the lines and the triangleswhich leads to the perception of the lines as being different in length.


Versions of the Sander Illusion

The following are some alternate versions of the Sander Illusion:

Sanders hybrid
Sanders hybrid



Illusions like the Sander Illusion

There are many other visual perception illusions that work in similar ways to the Sander illusion. Some examples include:

The Müller-Lyer illusion is a visual perception illusion that consists of two lines of equal length, with arrows at each end pointing either inward or outward. The line with the inward-pointing arrows appears shorter than the line with the outward-pointing arrows, even though they are actually the same length.

müller-lyer illusion
The Müller-Lyer Illusion

The Poggendorff illusion: This illusion is similar to the Müller-Lyer illusion, but instead of arrowheads, it uses a diagonal line that intersects two parallel lines. The brain perceives the diagonal line as being tilted, causing the parallel lines to appear as if they are not parallel.

Poppendorff Illusion Lines


The Ebbinghaus illusion: This illusion involves circles of different sizes, but with the same-sized circles placed close to each other. The brain perceives the smaller circles as being larger when they are placed next to larger circles.

Ebbinghaus Illusion


The T illusion: This illusion involves a T-shaped figure with a horizontal bar and a vertical bar. The brain perceives the vertical bar as being longer than the horizontal bar, even though they are the same length.

T Illusion


The Kanizsa triangle: This illusion is created by placing Pac-Man-like shapes around an invisible triangle. The brain perceives the triangle as if it were a real, visible triangle.

Kanizsa Triangle Illusion


The Zöllner illusion: This illusion involves parallel lines with diagonal lines intersecting them. The brain perceives the lines as if they were not parallel and that the diagonal lines are converging.

Zöllner illusion


Moon illusion: This illusion involves the perception of the Moon appearing larger when it is near the horizon compared to when it is high in the sky.

Moon Illusion


Ponzo illusion: This illusion involves the perception of an object appearing larger or smaller depending on the size of the surrounding context.

Ponzo Illusion


The Café Wall Illusion is a visual illusion that is created by a grid of alternating light and dark horizontal and vertical lines. The lines appear to be bent or tilted, even though they are actually straight.

Cafe Wall Illusion


All these illusions are based on the brain’s perception of visual context, depth cues, and grouping rules. These illusions help to understand how the brain interprets visual information and how it can be deceived.

Discovery of the Sander Illusion

The illusion is named after German psychologist Friedrich Sander who described the illusion in 1926. However, the illusion was published earlier by Matthew Luckiesh.

Matthew Luckiesh was an American engineer and scientist who made significant contributions to the fields of lighting and optical engineering.

He was born in 1883 and died in 1967. He is known for his work on visual perception and the development of advanced lighting techniques, including the “daylight lamp” and the “sun lamp.”

He also wrote several books on the subject of light and vision, including “Artificial Light: Its Influence upon Civilization” and “The Science of Light.”

Luckiesh was a leading expert in his field and was considered one of the most important figures in the development of modern lighting technology.


References and Resources

Check out our complete list of illusions.


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