Poggendorf's Illusion

Poggendorf's Illusion is an illusion that causes the viewer to misalign one piece of a diagonal line with it's match on the otherside of an ocluding box.

In all the cases below, the orange line connects to the green line on the other side.
0 degree tilt 10 degree tilt
20 degree tilt
0 degree tilt
10 degree tilt
20 degree tilt
30 Degree tilt 40 Degree tilt
50 Degree tilt
30 degree tilt
40 degree tilt
50 degree tilt
60 Degree tilt
70 Degree tilt
80 Degree tilt
60 degree tilt
70 degree tilt
80 degree tilt 

As you can see, as the angle of tilt increases, the your ability to match up the orange line with the green line decreases. By adding a second line to the image, the effect is diminished. and one can properly match up the orange and green lines. (4) What's interesting is that viewers still continue to see the illusion even after the correct line up is shown to them.

Click here for an explaination why!