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How You Judge Distance

by Nancy S. Holliday

If you want to hit a home run, you have to be able to see how far away the baseball is as it crosses home plate. Your brain has to take a measurement. How does it do it?

This experiment will show you. Use string to hang a paper clip or other small object in a doorway at about chest level. You may need help from an adult to hang it safely.

Pick up a stick, such as a yardstick or broom handle. Take three or four steps away from the paper clip.

Now close one eye and walk toward the paper clip. Try to touch it with the tip of the stick. If you miss, try again, still keeping one eye closed.

Using only one eye, most people will have a hard time touching the paper clip on the first try. Try it again with both eyes open and see if it’s easier to do.

How It Works
In the experiment, you have to judge how far away the paper clip is. Your brain has several ways of doing this.

One simple way depends on how much your eye has to focus. Even with only one eye, this way gives the brain some information about distance, but not much.

As the experiment shows, your brain can judge distance much better when you look at something with both eyes. In fact, your brain has more than one way to use both eyes in judging distance. For example, to see anything clearly, you have to make both of your eyes point at the object. When you look at a nearby object, your eyes turn inward, toward each other. You can feel this happening if you look at the tip of your finger and move your finger toward your nose.

Your brain gets information from the eyes about how much they are pointing toward each other. Using that information, the brain can tell how far away an object is, whether it’s a baseball or a paper clip.

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