• Background
  • Instructions
  • Illustration
  • Quiz


The Ponzo illusion is a strong example of misapplied size constancy as well as the influence of linear perspective (a monocular cue to depth) on size perception. The Ponzo illusion is illustrated below. The two lines are the identical size. They take up exactly the same amount of size on the page or screen. If you do not believe this, you can measure them. However, the line closer to the top of the page looks bigger. This is because there the side lines indicate depth through the depth but of linear perspective. We see the roughly parallel lines of the side receding into the distance. The Ponzo illusion is so called because it was discovered by the Italian psychologist Mario Ponzo (1882–1960).

Illustration of the Ponzo Illusion.

Use this activity to try the Ponzo Illusion and change several parameters about the illusion to see how impacts the strength of the illusion.


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab


Below is a list of the ways that you can alter the illustration. The settings include the following:

Top Line Length: adjust the length of the top line to make it appear the same length as the left hand line.
Match: if you think the two horizontal lines look the same length, click here to remove the vertical or near vertical lines to see if the lines still appear the same length.
Angle: change the angle two vertical or near vertical lines.
Test Line Position: adjust how far apart the two horizontal lines are.
Line Thickness: adjust the thickness of the lines makeing up the illusion.
Dashed Lines: select to make all of the lines dashed.
Separation: adjust to make the vertical or near vertical lines to be closer or farther apart.
Make the Same Length: For the two horizontal lines to be the same length.


Pressing this button restores the settings to their default values.