Both lines are exactly the same length.
Müller-Lyer Illusion
The arrow of size: why does one line look dramatically longer than the other?
🧐 What do you see?▼
Compare the two lines. The line with the outward-pointing arrows (fins) likely looks significantly longer than the line with inward-pointing arrows.
Both horizontal lines are exactly 200 pixels long.
🧠 Why this works▼
There are two main theories for the Müller-Lyer illusion:
- Size Constancy: Our brains interpret the "outward" fins as the corner of a room pointing *away* from us, and "inward" fins as a corner pointing *toward* us. If both are the same size on the retina, the "farther" one must be physically larger.
- Centroid Processing: The visual system averages the position of the fins with the line, literally "stretching" the perceived center of the object.
🧪 Try variations▼
- Direction: Toggle the Direction control to switch between horizontal and vertical lines. The illusion works perfectly in both orientations!
- Intensity: Adjust the intensity to change the angle of the fins. At sharp angles, the effect is usually much stronger.
❓ FAQ▼
Does everyone see this?
Interestingly, some cultures that live in non-"carpentered" environments (without square houses) show much weaker susceptibility to this illusion, supporting the room-corner perspective theory.