The Ponzo illusion is an optical illusion that was first demonstrated by the Italian psychologist Mario Ponzo (1882-1960) in 1913. He suggested that the human mind judges an object's size based on its background.
The typical example of this is the figure below. This is the typical railroad track scheme usually used to represent this illusion. The vertical lines appear to go off into the distance like train tracks. This gives us the impression that the line in the distance is larger then the line that appears to be nearer to the viewer.
Seeing this illusion against a great real photo backdrop can be amazing. Most of the time people pick train tracks. Below is an outstanding photo that presents this optical illusion slightly different...
The two red lines (or three if you move your mouse over the image) below are duplicates, they are the exact same size. Which image is more convincing the one with two lines or the one with three lines?
(c) 2006 Walt Anthony
Project: Create your own illusion. Here is a suitable image of train tracks (right click and select "save target as" or "save link as" depending on your browser). You can use Windows Picture and Fax viewer to print it out.
Now that you have it printed try and create your own illusion.
Use 2 Popsicle sticks painted red or just draw red lines with a marker.
Does changing the color change the effect?
Does using two different colors alter the illusion?
Try fat and skinny lines. Does the thickness of the lines affect this illusion?
Vary the distance between the lines, does this change the effect?
Is a three dimensional Popsicle stick as effective as 2 dimensional red line?
Does the distance from which you view the illusion change the illusion?
What can you conclude, if anything, about how we perceive things from this project?