Ebbinghaus Illusion

Materials: ★☆☆ Easy to get from supermarket or hardware store
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★☆☆ Minimal safety procedures required

Categories: Senses and Perception

Alternative titles: Relative Size Illusion

Summary

The Ebbinghaus Illusion shows how surrounding context changes our perception of size. Two identical central circles appear different in size when one is surrounded by larger circles and the other by smaller circles.

Procedure

Draw or project two identical central circles.
Surround one central circle with several larger circles placed close around it.
Surround the other central circle with several smaller circles placed close around it.
Ask observers which central circle looks bigger.
Reveal that both central circles are exactly the same size.

Links

The Ebbinghaus Illusion | Optical Illusions - Contact Lens King:

Explaining Optical Illusions: Part 4 - The Ebbinghaus Illusion and the Flash lag effect - Learning Curve:

📄 Ebbinghaus illusion - Wikipedia: https://en.wikipedia.org/wiki/Ebbinghaus_illusion

Variations

Change the distance between the central circle and the surrounding circles to see if the illusion becomes stronger or weaker.
Use different colors or shading for the circles to test if contrast affects perception.
Replace circles with other shapes (e.g., squares or triangles) to see if the illusion persists.
Present the illusion digitally and allow students to adjust the size of one circle until they believe both are equal, then measure.

Safety Precautions

Ensure that projections or printed images are clear and visible to avoid eye strain.
If drawing by hand, use safe tools (compass, rulers, markers) with caution.

Questions to Consider

Why does one central circle look larger even though they are the same size? (Because the brain judges size relative to surrounding objects.)
How does this illusion demonstrate the role of context in visual perception? (It shows that perceived size is influenced by comparison to nearby objects rather than actual size alone.)
Would the illusion still work if the surrounding circles were farther away from the central circle? (The illusion would be weaker.)
What real-world examples might relate to this illusion? (Perceiving object sizes in crowded environments, advertising layouts, or optical effects in design.)