Ponzo Illusion

Named for an Italian psychologist, the Ponzo Illusion is the mind's tendency to judge an object's size according to the background. An example of this is a set of converging lines drawn upward with an equal set of parallel lines drawn horizontally (this diagram appears to be like railroad tracks that stretch off into the distance). The horizontal lines (the railroad tracks in our example) near the bottom where the converging lines are farther apart appear to be shorter, and the rungs near the top where the lines are closer together appear to be longer. At the bottom, the horizontal tracks stay inside the vertical lines while the top ones extend over them. It is this difference in background that makes the top tracks appear longer.