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Hole in the Hand

In this activity students learn how the separate images from each eye can combine to create the illusion of a hole in the hand.

If you close your left eye, what do you see?

If you close your right eye, what do you see?

The two images you saw were probably pretty similar. Your brain knows that and is able to put the two images together and make one single image. With two eyes, we have a wider field of vision and can see more than we could with just one eye.

In a 3-D world, having two eyes helps us figure out how far away something is. Our eyes need to focus differently in order to see things at different distances.

To see something very far away, our eyes almost point straight forward. To see things really close, our eyes will have to slightly turn toward each other. These different focusing abilities help our brain figure out what is far away and what is really close.

Objectives

  • Describe how the brain helps us create images from what we observe.

Materials

  • Per Student:
    sheet of paper

Key Questions

  • Why does there appear to be a hole in your hand?

What To Do

  1. Ask the students if any of them have X-ray vision and then tell them that they all do.
  2. Give each student a sheet of paper and tell them to roll it into a tube. The diameter of the tube should be about 2.5 cm/1 in (or at least large enough to stick 2 fingers into).
  3. Tell the students to hold up their left hand in front of them and place the tube between the index finger and the thumb.
  4. Tell students to move the tube up to their right eye to look through it like a telescope but keep there left eye open.
  5. Ask students to explain what is happening.

Explanation: Normally, both of your eyes see the same thing, just from slightly different angles. Your brain combines these two slightly different views to let you see in three dimensions and judge distances. This experiment changes that. You are giving your brain two different images. One eye is seeing the palm of your left hand. The other eye is seeing the other side of the room, viewed through the tube.

In combining the two images, your brain has to decide what is more important to see. Most of the view from your right eye is blocked by the dark sides of the tube, so you mostly see the view from your left eye. The one exception is the bright circle of image that your right eye sees through the tube. Since this is the one bright spot from your right eye, your brain pays extra attention to it. As your brain combines the two images, you wind up seeing your hand with a hole through it.

Extensions

  • If you want to play games with your brain, try watching television with the ‘X-ray tube’. What happens?
  • Have someone stick their finger into the end of the tube. What happens?
  • Play depth perception games like the Penny Drop.