Ponzo's illusion is the apparent difference in length of two equal line segments placed between converging lines. In the figure above, the upper horizontal line segment often appears longer than the lower one, although both have the same length.
In this schematic railway-track version, the two orange bars also have equal lengths. The converging rails suggest receding distance toward a vanishing point, which can make the upper bar appear longer. The effect can also be produced using vertical bars (Seckel 2000, illusion 85, p. 97).
