Dec. 24, 2003

Discover How Your Pupils Work

You can discover how your pupils work with the help of a friend.

Turn out the lights in a room so that it is dim enough that you can still see. After a few minutes, look into the eyes of your friend and see how large the pupils are.

Now turn on the lights or shine a flashlight into your friend's eyes. Notice how quickly the pupils close. When you remove the light, do the pupils get large again?

This automatic adjustment is called a reflex. A reflex is an instant response to something, and it happens without thinking. The pupil reflex was discovered in 1751. It was the first of many reflexes all over the human body to be discovered.

Here's another way to experience your pupil reflex.

Stand near a light switch about 6 feet (2 meters) away from a television set or computer monitor that is on. Turn off the lights and stare at the brightly lit screen. After about three minutes, turn on the light. The screen will suddenly appear dim.

Here's what just happened. In the dimly lit room your pupils open wide. Most of the light in the room comes from the screen. When you turn on the light, your pupils get smaller to adjust to the brightness. As a result, less light reaches you from the screen, and it appears to get dimmer.

Reprinted with permission from Open Your Eyes: Discover Your Sense of Sight by Vicki Cobb. Text © 2002 by Vicki Cobb (www.vickicobb.com). Published by Millbrook Press.

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