Poggendorff Illusion


In the figure at left, the two diagonal line segments appear to be offset from each other, but they are actually part of the same line (click on the reveal button). In fact, the illusion is so strong that it is often taken into account when designing logos and patterns. For example, although the diagonal segments in the Union Jack (the flag of the United Kingdom, shown at right) are not aligned, a pair of them appears as though they are.


[1] M. Bach, "Poggendorff Illusion," Jun. 22, 2002. [Online]. Available: http://www.michaelbach.de/ot/ang_poggendorff/index.html. [Accessed: Mar 3, 2010].
[2] J. D. A. Wiseman, "How to draw the Union Jack — accurately," Nov. 1999. [Online]. Available: http://www.jdawiseman.com/papers/union-jack/union-jack.html. [Accessed: Mar 3, 2010].