___ / b
| |// c
Can you see which line to the right is the continuation of the line to the left?
The Poggendorff illusion is an optical illusion similar to the Herring Illusion, the Muller-Lyer Illusion and the Zollner Illusion, showing how lines can appear distorted due to their background (or in this case, foreground). This particular illusion is one that our brains can be trained not to see, in other words the illusion is stronger the first time you see it.