Key concepts
Optical Illusions

Have you ever stopped to wonder why you have two eyes—but only see one image? Usually it is because your brain takes the information from each of your eyes and combines them, without you even noticing! But sometimes your brain is too smart for its own good; it makes assumptions (or guesses) about the things you see. When your brain makes guesses it sometimes makes mistakes, as it will do in this activity!

Why do you need two eyes to make one image? It turns out that you actually don't need two eyes for that—you can close one eye and still see pretty well. But having binocular vision (two eyes working together) has some advantages. For instance, binocular vision gives you much better depth perception, increases the size of your visual field and improves the accuracy of your vision.

When you want to look at something in front of you, you focus your eyes so that they are both pointing toward that object. (If the object is very close to your face, you may even go a little cross-eyed!) The cells at the back of your eyes—in their retinas—send signals to your brain about how much light and color they see. Your brain processes these signals to determine information such as the object's shape, distance from your face and location in your visual field as well as the amount and direction of light in the field. By establishing these parameters for the object you are looking at, your brain is able to combine the information coming from your left and right eyes into one cohesive image.

Your brain is good (and fast) at what it does because it is designed to make intuitive assumptions. For example, your brain assumes that your eyes are focused on the same thing in your visual field. This is a very smart guess for your brain to make, because it is almost always true.

For this activity you are going to trick your brain by having your eyes send different information about what you are seeing. When your brain tries to combine the images it receives from your left and right eyes, it will come up with some pretty interesting results!


  • A piece of white paper, eight and one half by 11 inches
  • Clear tape


  • Roll the paper along the longer side into a tube; the tube should be about the diameter of a quarter.
  • Use a piece of tape to hold the paper in place.


  • To start, choose a background to look at (a wall, a door, etcetera) that is not white. And be sure you are in a well-lit space. This activity works best with a nonwhite background, in a bright area.
  • With both eyes open, raise your right hand so that your palm is facing toward your face, about one foot away from you.
  • Look at your hand with both eyes open. Then use your left hand to alternately cover your left eye, then your right eye while you continue to look at your right hand through the uncovered eye. Do this slowly at first. As you slowly switch eyes, what do you notice first, each time you switch which eye is uncovered? Do you notice what is behind your hand? Do you notice the lines on your hand? (Just for fun: If you switch eyes fast enough, it may look like your right hand is moving back and forth a tiny bit!)
  • Open both your eyes and keep them open. Lower your right hand, and using your left hand, place the paper tube up to your left eye (being careful with any sharp paper edges) so that you are looking through it as you would a telescope. What do you see? Do you notice the paper tube first or the hole at the end of it? What do you notice about what your left eye sees compared with what your right eye sees?
  • Raise your right hand so that your palm faces toward you, then place it against the tube so that the outside of your pinky finger is touching the tube, about halfway down its length. Look straight ahead with both eyes open. What do you see? Do you notice anything about your right hand? Can you change what you see by moving your right hand closer or farther away from your face? What about if you move your right hand closer or farther away from the tube?
  • Keeping the tube and your right hand in place, try closing your right eye, then your left eye. (You may need a helper for this step if it is difficult to wink both eyes on your own.) What is different about what your left and right eyes are seeing? What is the same?
  • If you saw something strange, try this: Without moving your right hand, slowly move the tube away from your face. See if you can continue to see the illusion, even as the tube gets farther away. How far away does the tube have to be before you stop seeing the illusion?
  • Test what happens if you repeat this activity but this time look through the tube with your right eye. Does this work for you with one eye but not the other or does it work for both eyes?
  • Extra: Ask your parents and friends to try this activity and see if the illusion works better if they use their left or right eyes to look through the tube.

Observations and results
When you looked through the tube, did you see a hole in the hand pressed up against the tube?

So why do you see the hole in your hand? Whenever your eyes are open, your brain is working to combine the information coming from your left and right eyes into one image. This usually works remarkably well, and you don't even notice because both your eyes are usually looking at the same thing. In this activity, however, you are changing that one little fact: Your left and right eyes are seeing two different things! Your left eye is seeing a small circle of the world at the end of a tube whereas your right eye is seeing your right hand.

When your brain combines the information coming from the left eye and the right eye, it looks something like this: Small circle of the world (from left eye) + right hand (from right eye) = small circle of the world going through your right hand!

If you tried the activity with both eyes, did you notice that one eye worked better than the other? Some people have a "dominant eye," similar to how they have a dominant hand. If you have a dominant eye, information coming from the dominant eye will take precedent over data from the nondominant eye. In this case the tube illusion will work better when the person is looking through the tube using their dominant eye.

More to explore
Why Do We See in 3-D?, from Live Science
Now You See It, Now You Don't: A Chromatic Adaptation Project, from Science Buddies
Seeing Science: Solving the Mystery of the Shrinking Moon, from Scientific American

This activity brought to you in partnership with Science Buddies

Science Buddies