This is one of the most well-known illusions. The angles, or arrows, influence the apparent length of the two line segments, so that the left segment appears considerably longer than the right segment. You can change the orientation of the arrows with the Flip Left Arrow, Flip Middle Arrow and Flip Right Arrow commands to see how the arrows affect the apparent lengths of the line segments. The line segments remain exactly the same length.
References
Coren, S. & Girgus, J.S. (1978). Seeing is Deceiving: The Psychology of Visual Illusions. Hillsdale, New Jersey: Lawrence Erlbaum Associates.
Luckiesh, M. (1965). Visual Illusions, Their Causes, Characteristics and Applications. New York: Dover.