Filled Space Illusion

In the filled space illusion (or Oppel–Kundt illusion) The line B is equidistant between A and C but appears closer to A.

This is because the filled space between B–C appears larger than the unfilled space between A–B.

Oppel-Kundt Filled Space Illusion
Filled Space Illusion

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How does the Filled Space Illusion work?

The Oppel–Kundt Illusion is also known as the filled space illusion. It a visual illusion where an enclosed area appears to be larger or smaller than it actually is, depending on the amount of detail or texture within the area. The illusion is created by the brain’s perception of the amount of “visual mass” within the enclosed area.

For example, in the filled space illusion, an enclosed area filled with a lot of detailed or textured elements will appear smaller than an enclosed area with less detail or texture. This is because the brain perceives the detailed area as having more “visual mass” and thus it appears smaller. Conversely, an enclosed area with less detail or texture will appear larger, as the brain perceives it as having less “visual mass”.

This illusion works by the brain using the texture and details to make judgments about the size of an object, rather than relying solely on the object itself. This is similar to how the brain uses the surrounding elements in the Helmholtz illusion to make judgments about the size of an object.

It is also related to the concept of “figure-ground” perception, in which the brain organizes visual elements into distinct foreground and background regions, and the perception of the size of an area can be affected by the surrounding elements and how they are perceived as figure or ground.

Versions of the Filled Space Illusion

The following are a alternate versions of the Filled Space Illusion:

Oppel-Kundt Filled Space Illusion


Oppel-Kundt Filled Space Illusion


Illusions like the Filled Space Illusion

The following are some illusions similar to the Filled Space Illusion.

The Helmholtz Illusion. The square of horizontal lines appears to be taller and narrower than the identical square of vertical lines.

Helmholtz Illusion

The Zöllner illusion is a visual illusion in which parallel lines appear to be angled due to the presence of intersecting lines.

The illusion is often used to study the brain’s perception of shape and spatial relationships. It is considered one of the most powerful and striking examples of a geometrical-optical illusion. The perception of the illusion can be explained by the brain’s tendency to group lines together based on their similarity in direction and spacing, which can lead to an overestimation of the angle between the parallel lines.

Zöllner illusion


The Müller-Lyer illusion: This illusion involves lines with arrowheads at the ends, which can make a line appear longer or shorter depending on the direction of the arrowheads.

müller-lyer illusion



The Poggendorff illusion is a visual illusion in which the brain perceives a diagonal line as being interrupted by an object, even though the line is actually continuous. The illusion is created by the misalignment of two parallel lines that are intersected by a third line at a certain angle.

Poggendorff illusion transparent gray bar



The Orbison illusion is an illusion of movement that is created when stationary, parallel lines are placed on a background of radiating lines. The parallel lines appear to be moving in the direction of the radiating lines, even though they are actually stationary. This illusion is thought to be caused by the interaction between the parallel lines and the radiating lines, which creates an illusion of movement in the brain.

Orbison Illusion



The Wundt illusion is an optical illusion produces an inversed effect compared to the Hering Illusion. The vertical lines are both straight, but they may look as if they are curved inwards.

Wundt Illusion

The Cafe Wall Illusion is a geometric optical illusion that is created by the alignment of parallel lines in a checkerboard pattern. The parallel lines appear to be tilted or slanted, even though they are actually straight.



Discovery of the Filled Space Illusion / Oppel–Kundt Illusion

The illusion was described by German psychologist Karl Oppel and later independently by German physicist August Kundt.

Karl Oppel was a German psychologist who lived in the 19th century. He is best known for his work on visual perception, specifically for his description of the Oppel–Kundt Illusion in 1875. He was a professor of psychology at the University of Breslau.

He made significant contributions to the field of perception and visual cognition and his work on the Oppel–Kundt Illusion was considered an important early example of how context and surrounding elements can influence our perception of the world. His work laid the foundation for much of the research on visual perception that followed.

Karl Oppel also made important contributions to the field of sensory perception, particularly in the areas of touch, hearing and vision, as well as to the area of experimental psychology, where he developed techniques for measuring and analyzing sensory perception.

His work was widely recognized and respected in his field, and he was a member of several professional organizations, including the German Society for Psychology, the German Society for Anthropology, Ethnology and Prehistory, and the German Society for Experimental Psychology.

August Kundt was a German physicist who lived in the 19th century. He is best known for his work on the nature of sound and light, and for independently discovering the Oppel-Kundt illusion in 1877. Kundt’s discovery of the illusion was made independently of Karl Oppel, who had described a similar illusion three years earlier.

Kundt was a professor of physics at the University of Strasbourg, where he conducted research on a wide range of topics in physics, including the nature of sound and light, the properties of gases, and the nature of electricity and magnetism. He made important contributions to the field of acoustics, particularly in the areas of sound wave propagation and the study of musical instruments.

He also made significant contributions in the field of optics, where he developed a method for measuring the refractive index of a material by measuring the speed of light in that material. He also worked on the phenomenon of dispersion and the study of color, which helped in the development of the theory of color vision.

Kundt’s work was widely recognized and respected in his field, and he was a member of several professional organizations, including the German Physical Society and the Royal Society of Sciences in Heidelberg.

References and Resources

Check out our complete list of illusions.

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