You can draw your own simple stereogram. Draw two identical rectangles next to each other. Draw a small circle in the center of the one on the left. Draw an identical circle slightly to the left of center in the rectangle on the right. Hold the paper at arm's length or tape it to the wall and cross your eyes slightly so that the two squares exactly overlap. The circle will appear to float above the paper. See if you can construct more complex three-dimensional pictures.

How do your eyes work? Why do you have two of them and why are they placed where they are on your head?

Why do some people need glasses? Why do some people need them for distance and some need them for reading? How do optical illusions trick your eyes into seeing things you're not?

Overview
Look around. What do you see? Human beings can obtain a large amount of information about the surrounding environment through their sense of vision. But to see, we need light and the light-processing organs called eyeballs.

The outside of an eyeball is white, except for the clear, bulging cornea in front. Just behind the cornea is the iris, a colored area with a hole in the center called the pupil. Circular muscle tissue in the iris allows it to open and close the pupil to regulate the amount of light that gets inside the eyeball. Just behind the iris and pupil is the lens. The cornea and the lens work together to focus images on the retina, the light-sensitive layer that lines the inside of the eyeball.

Light moves in straight lines. Whenever a light ray encounters a surface of a different transparent medium, however, it bends (refracts) and heads off in another direction. The amount of bending depends on the nature of the transparent substance, the angle at which the light hits the surface, and the color of the light. On a curved surface such as a lens, parallel rays of light will hit the surface at different angles and will be bent differently. A greater curvature will lead to a greater difference in the amount of bending.

When your eye focuses on an object, all the light rays from a single point on that object are bent toward a single point on your retina. In the eyeball, light rays passing through the cornea are bent by its curvature toward the pupil. The lens flexes to change its curvature and finish the focusing process. Interestingly, the image projected on the retina is upside down because of the way the rays of light are bent by a double-convex lens.

On the retina are two kinds of cells that change light into nerve impulses. Rod cells do not see color but are best for night viewing because they react to very low light levels. Cone cells are for color viewing. They work best in good light and are found mostly in the center of the retinaÐan area called the macula, which provides the sharpest vision. Within each eye is a small blind spot with no rods or cones, where the optic nerve is attached to the eyeball. The optic nerve collects the nerve impulses and carries them to the brain, which interprets them as an image.

Connections

1. Many animals besides humans use vision. Are there any animals that do not have or use vision?
2. Two eyes are needed for "stereoscopic" or 3-D vision. Why do you think this is so? Do all animals that see have two eyes? Why are some animals' eyes on the side of the head rather than in the front?

Resources

Minnaert, M.G.J. (1993) Light and Color in the Outdoors. (Translated and revised by L. Seymour.) New York: Springer-Verlag.

Parker, S. (1989) The eye and seeing. In the Human Body series. New York: Franklin Watts.

Simon, H. (1983) Sight and seeing: A world of light and color. New York: Philomel Books.

American Science and Surplus
3605 Howard St
Skokie, IL 60076
(847) 982-0870
(lenses and other optic equipment)

Flinn Scientific, Inc.
PO Box 219
Batavia, IL 60510-0219
(800) 452-1261
(science equipment for schools)

All about your eyes
http://www.ushc.com/health/3109.html

An index to vision sites is available at "vision science" http://vision.arc.nasa.gov/VisionScience/VisionScience.html

Main Activity

As an object approaches, the human eye's lens flexes to focus on it. Eventually the object gets so close, however, that the lens can no longer focus on it. Then the object begins to blur. How close can you bring an object before it looks blurry? Does this distance vary for different people or age groups? Does the shape or color of the object make any difference? Does it matter how brightly the object is illuminated?

Materials

• a 2.5-cm x 5-cm (1"x 2") swatch of printed words from a newspaper or magazine
  
• modeling clay or sculpting compound
  
• a 3" x 5" index card
  
• a cloth or soft vinyl tape measure like those used in sewing (CAUTION: Do not use any sharp or pointed objects, including wooden or plastic rulers, since these materials will be held close to students' faces.)
  

1. Glue or paste the newspaper or magazine selection in the center of the 3" x 5" card.

2. Roll the clay into a 5-cm (2") ball and mount the 3" x 5" card in it.

3. For the first test, have the test subject cover one eye with a hand.

4. Slowly bring the clay ball and words directly toward the test subject's uncovered eye. The test subject should try to focus on the words.

5. The test subject should say "stop" when she or he can no longer focus clearly on the words. Stop moving the ball at that point.

6. Have the test subject hold one end of the tape measure to her or his cheekbone just below the eye and measure the distance to the 3" x 5" card. (NOTE: Having the test subjects measure the distances helps ensure that no eyes get poked.)

7. Write down the measurement. Be sure to include whether or not the test subject wears glasses or contact lenses.

8. Repeat the test several times, using different test subjects and testing different variables. For example, try the test with both eyes uncovered, with and without glasses, with different amounts of light, and so on. Just remember to change only one variable for each test and to repeat each test at least once. Average the results of repeated tests.

Questions

1. What is the average distance where the image begins to blur for all test subjects? Is the average distance larger or smaller for people who wear glasses? Is it larger or smaller for one eye or both eyes? Is the distance the same for both eyes of the same person?
   
2. Can you design another experiment that tests how wide a person's field of vision is? How large an arc the blind spot covers?
   

On a dark evening, go outside into your backyard and try to look at familiar objects. Can you see them better if you look straight at them or if you look just to the side of them? Why do you suppose that is?

Change the color scheme on a color television. Try different colors. Try black and white. Can you see as much detail on the black-and-white screen? (Ask your parents for permission to do this.)

One evening, stand in front of the mirror in the bathroom at home. Turn off the light for several minutes. When you turn it back on, observe the iris in your eye. How fast does it change shape to react to the light?

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