Zollner illusion. The horizontal lines are parallel but appear to tilt alternately, i.e., the acute angles formed by the horizontal lines and the short inducing lines appear to expand. It is said that the illusion is maximum when the intersecting angle is 10 - 30 deg. ... The oblique arrangement increases illusion.

Why does the Zöllner illusion happen?

The shorter lines are on an angle to the longer lines, and this angle helps to create the impression that one end of the longer lines is nearer to the viewer than the other end. This is similar to the way the Wundt illusion appears. It may be that the Zöllner illusion is caused by this impression of depth.

What is Zöllner illusion in psychology?

a visual illusion in which parallel lines appear to diverge when one of the lines is intersected by short diagonal lines slanting in one direction, and the other by lines slanting in the other direction. [