Poggendorff Illusion
The offset line: why do we lose track of symmetry behind a simple box?
🧐 What do you see?▼
Look at the black line segments on either side of the dark rectangle. Do they look like they are part of the same continuous line?
To most people, the segment on the right looks higherthan the segment on the left. In reality, they are perfectly aligned.
🧠 Why this works▼
Discovered by Johann Poggendorff in 1860, this illusion happens when a diagonal line is partially blocked by an object.
The brain has trouble with angular intersections. When the diagonal line passes behind the rectangle, the brain "overestimates" the acute angles at the intersection point. This miscalculation causes the perception of the line to shift vertically, making the segments look disconnected.
🧪 Try variations▼
- Pause to Verify: Hit the Pause button! A green dotted helper line will appear to show you exactly how the lines align.
- Intensity: Use the Intensity slider to change the width of the rectangle. As the rectangle gets narrower, the illusion usually weakens.
❓ FAQ▼
Does this happen in the real world?
Yes, architects have to be careful when designing slanted roofs or bridges that pass behind support pillars, as they can look physically "broken" to observers from certain angles.