This is the Ponzo illusion, named after the Italian psychologist Mario Ponzo who first demonstrated it in 1913. This illusion is a classic example of a geometrical-optical illusion, revealing how our perception of size can be influenced by the perceived depth or distance in a scene.
In the Ponzo illusion, two horizontal lines of identical length are drawn over a pair of converging lines, which are often depicted as railroad tracks extending into the distance. Due to the perspective cues provided by the converging lines, we interpret the scene as representing a three-dimensional space. In this perceived 3D space, the upper line seems to be further away compared to the lower line.
Interestingly, our visual system expects objects further in the distance to be smaller in their physical size, as they typically project a smaller image onto our retinas. However, in the Ponzo illusion, the two horizontal lines are the same size in the 2D image. This discrepancy between the expected and actual size of the upper line causes our brain to overcompensate and perceive the upper line as being longer than the lower one, even though they are the same length in the image.
This illusion highlights the principle of size constancy, where our brain adjusts the perceived size of an object based on its perceived distance to maintain a consistent perception of the object’s size. In the case of the Ponzo illusion, this mechanism leads to a misinterpretation of the line lengths due to the misleading context of the converging lines, showing that our perception can often be skewed by contextual cues and the perceived depth or distance in a scene.