Our eyes work in a way that is similar to a camera. Invite students to discover this using the Parts of the Eye Reproducible, below. Begin by talking them through the process: 1. Light rays travel from an object to the eye, and become refracted (bent) by the cornea. 2. This refracted light passes through the pupil to the lens, where the light is bent even more.
3. The light then passes through the vitreous humor to form an image on the retina.
4. Electrical signals are sent to the optic nerve.
5. The image is carried to the brain and processed.
Once students are familiar with the concepts, challenge them to read the word list and label all the parts of the eye.
Eye on the Image
When the retina receives an image, the image is actually upside down. You can illustrate this point for students with a simple demonstration. Place a small television directly in front of you and roughly at waist height. Turn on the television, turn off the lights, and stand about 10 feet from the screen. Hold up a magnifying glass to a sheet of white paper. Slowly move the magnifying lens toward and away from the paper until you see a clear, upside-down image of the television, as shown. Explain to students that what is happening is very similar to what goes on in an eye. Light leaves the screen and spreads all over the room. When light rays hit the magnifying glass, the glass acts just like the lens of an eye. It refracts the light rays, bending them together into a single point. At the same time, the glass also inverts (flips upside down) the rays. Then all the points of light from the screen are recombined into a single inverted image on the white paper, just as the things we look at appear on our retina. Give each student a chance to do the demonstration, and have them identify which parts of the eye the glass and paper represent.
"Eye" Spy a Cat
Give each of your students a sheet of blank white paper and a copy of the Black Cat Reproducible, below. Have them cut out the cat picture. Next, have students stare at the cat for 45 seconds, then stare at the blank sheet of paper. What do they see? Can anyone explain the “mystery cat” that appeared on the paper? Answer: The retina at the back of the eye contains light-sensitive cells. When light from an image reaches these cells, electrical signals are sent to nerve fibers, then to the optic nerve and, finally, to the brain. When you stare at an image for a long time, signals are fired to the brain over and over, even after you have stopped looking. Thus, your brain thinks it is still “seeing” the image.
Pair off students with flashlights and mirrors. Have them examine their pupils, then turn out the lights. Instruct students to take turns holding a flashlight below their chins, then have them look in the mirror again. Are their pupils larger or smaller when they are in the dark? Why? Explain to students that there are two sets of muscles that make up the iris. In bright light, radial muscles on the outside of the iris contract the pupil to restrict the light that enters the eye. In dim light, circular muscles on the inside of the iris enlarge the pupil, allowing in more light. Charting Vision Why do some people need glasses? Just like an out-of-focus camera lens, a person's vision becomes blurry when the eye's lens does not properly bend light into a clear image on the retina. Eye doctors use a letter diagram called a Snellen chart to test for this. Invite students to choose letters to make their own inventive eye charts, using real ones as models (visit www.i-see.org/eyecharts.html for examples). Then have student pairs check each other's vision.
Focus on Safety
Invite your school nurse to give a talk about eye safety, then have students create posters to share what they've learned. The discussion could include such topics as when to go to the eye doctor, what to do if you get something in your eye, what to do if you injure or scratch your eye, how to avoid pink eye, and when and why to wear safety goggles. Have each student choose a topic, then write and illustrate a poster with drawings or magazine cutouts. Younger students can each make simple mini-books on eye safety.
“Seeing With Your Hands”
Begin this activity with some thought-provoking questions: What would life be like if you couldn't see? How would you identify an object if you couldn't see it? What other senses could you use? Then have students pair off. One child will be the “listener,” one will be the “toucher.” Give each pair a set of interesting objects, such as a grapefruit, a rubber duck, a ruler, and a tambourine. Instruct the listener to close his or her eyes. Challenge the toucher to describe the object using descriptive words but excluding those based purely on vision. For example, he or she might describe a grapefruit as “round” and “fruity-smelling,” but not as “yellow.” Encourage students to be creative in their descriptions. The listener should try to guess the object. Which objects were hardest to describe? Which were the hardest to guess?
Invite students to learn about Braille, a system of writing for the blind that uses raised dots. Begin by distributing copies of the Braille alphabet, available at www.nbp.org Each student will also need handfuls of split peas, construction paper, and glue. After students have created their names in Braille, encourage your class to try reading one another's names with their hands.
Which eye color is most common in your classroom? Just by “eyeballing” it, can students guess how many of their classmates have brown eyes? Hazel? Green? Blue? Have them jot down their estimates, then distribute the Eyeball Reproducible, below. Ask students to cut out the eye picture and color it in using a crayon that best matches their own eye color. Then create a class bar graph by calling students to the board; have brown-eyed students use their paper eyes to make the first bar, then have hazel-eyed students make the second bar, and so on. How close were students' original estimates?
For older students: Use this activity as an introduction to an online research study of dominant and recessive genes, which control iris color. For example, it takes just one brown-eye gene to result in brown eyes, but it requires two blue-eye genes to get blue eyes.