Mystery of the Senses—Vision

Student Handout

Optical Illusions

Scientists study optical illusions to learn how the visual system of our eyes and brain operates. Some kinds of illusions work because of the way our eyes and brain process images. Other illusions are based on the way we interpret two-dimensional images.


  1. Read the directions under each illusion, and write below it if you think the illusion works because of the way your brain process images or because of the way you have been trained to look at two-dimensional images. Explain your answers.

optical illusion, two faces/vase
Do you see two faces or a vase?
optical illusion with 2 lines
Does one line look longer than the other?
optical illusion with parallel lines
Do the long lines look parallel? Angle the page so that you're looking at the lines from the lower left corner. Now do they look parallel?
optical lillusion with letters
Can you see the letters or just the shadows of the letters?
optical illusion with squares
When you look at the white lines, do you see gray spots at the intersections?
optical illusion with a spiral
When you rotate this spiral does it seem to shrink or expand? When you stop rotating it, does it seem to shrink or expand?