What optical illusion makes parallel lines look curved?
Quick answer
The best-known illusion that makes straight, parallel lines appear to bend is the Hering illusion, described by German physiologist Ewald Hering in 1861. Two straight parallel lines drawn over a background of lines radiating from a central point appear to bow outward, like the sides of a barrel.
How the Hering illusion looks
Imagine a sunburst of lines spreading from the center of a page, like spokes. Draw two straight horizontal lines across it, one above and one below the center. Although they’re perfectly straight and parallel, they appear to curve away from the center. Placing a ruler along them proves they’re straight.
Why it happens
Scientists aren’t completely sure, but leading ideas include:
| Explanation | Idea |
|---|---|
| Angle distortion | The brain exaggerates small angles where lines cross |
| Motion prediction | The radiating pattern resembles what we see when moving forward; the brain may “predict” how the lines will look a split second later |
| Depth cues | The background suggests a 3D tunnel, changing how we perceive straight lines |
Similar line illusions
| Illusion | Effect |
|---|---|
| Wundt illusion | Lines over a pattern converging the other way seem to bow inward |
| Café wall illusion | Rows of offset black and white tiles make parallel horizontal lines look sloped; it was noticed on the wall of a café in Bristol, England |
| Zöllner illusion | Parallel lines crossed by short diagonal marks seem to tilt toward each other |
| Müller-Lyer illusion | Lines of equal length look longer or shorter depending on arrow-like ends |
What illusions teach us
Optical illusions show that we don’t see the world exactly as it is: the brain interprets signals from the eyes using shortcuts and assumptions. Light first lands on the retina; see what the retina is. Studying illusions helps scientists understand how vision works.
Related tricks of perception
- Seeing faces in random objects; see what pareidolia is
- A spinning wheel that seems to turn backward on video; see the wagon-wheel effect
- The Moon looking huge near the horizon; see why the Moon looks bigger near the horizon
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