Looking Into the Eye

Do you know how the human eye functions?

1 As you begin reading this, you probably aren’t thinking about how your eyes allow you to see the letters, the book, and everything else in the room. But your eyes are the complex organs that perform this very specific function, and without them you would not see. Your eyes change the light patterns surrounding you into information the brain can use to form a visual image.

2 Light is the key to vision. Light waves coming from the sun or a lightbulb bounce off objects and surfaces, creating unique patterns and colors for your eyes to detect. Light enters the eye first through the cornea, the tough, clear outer layer of your eyeball. The cornea protects the fragile parts located inside the eyeball. It is also the first place where light is refracted so that the image you see is in focus. When someone has laser eye-surgery, it is a defective cornea that is cut and reshaped so that the person can see clearly without glasses.

3 Directly behind the cornea is a watery fluid, called the aqueous humor, that keeps the moving parts of the eye flexible and hydrated. The iris, the colored part of your eye, is the first of these moving parts. The black dot in the middle of your iris is an opening called the pupil, and it controls the amount of light entering the eye. Muscles in the iris contract and relax to change the size of the pupil. When you walk indoors on a sunny day and notice that it takes time for your eyes to adjust, it is because you are waiting for your pupils to open wide enough to make up for the reduced light.

4 Human beings have round pupils, but many animals have pupils of different shapes. Cats and snakes, for instance, have slit-shaped openings in their irises, allowing them to see in a wider range of light and dark circumstances. No matter what their shape, pupils appear black because they are the hole leading into the dark interior of the eyeball where light is absorbed. 5 Sitting right inside the pupil’s opening is the lens, the other moving part of the eye. The lens is a flattened, transparent disc that works with the cornea to focus images onto the back of your eyeball. Although the shape of the cornea is fixed, the lens is attached to muscles that make it rounder or flatter, depending on what is needed for focusing.

6 Inside the eyeball is a clear, jelly-like substance that keeps it round, like air filling a balloon. The back wall of the eyeball contains a very important area called the retina. The retina is covered with two kinds of light-sensitive nerve cells. Six million of them are cones, the cells that read color. Coneshaped cells are divided into three types that detect only specific colors—yellow, green, or blue—but combine to let you see an incredible array of different colors. Color blindness is caused by a lack of one or more of these specific cone cells.

7 The most abundant cells in the retina, nearly 120 million of them, are the rods. These rod-shaped cells are much more sensitive to light than the cones, but they only produce black and white imagery. Most of the cones are located in the center of the retina, where you have the sharpest focus and sight. The edges of the retina, however, contain more rods that can detect very subtle amounts of light.

8 If you go out on a clear night, you might notice that the faintest stars are very hard to see when you look directly at them. Try looking slightly to the side of the star. The light-sensitive rods on the edges of your retinas will allow you to see the star more clearly!

Spectrum Reading Grade 6

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1. What do pupils control?

2. What is the colored part of the eye called?

3. If you walked from a dark room into a brightly-lit room, how would the size of your pupils change?

4. What colors do the cone-shaped cells in your eyes detect?

5. What kind of cells are missing from the eyes of a person who is colorblind?

6. Name two ways in which cone-shaped cells and rod-shaped cells are different from one another.

7. What is the purpose of the cornea?

8. What purpose would a reader have for reading this selection?

to form an opinion

to learn

for entertainment

9. Check the sentence in which pupil has the same meaning as it does in paragraph 3.

The pupil determines how much light is allowed into the eye.

The best pupil in the class received a perfect grade on her spelling test.

10. Check the sentence in which cells has the same meaning as it does in paragraph 6.

Bees’ honeycombs contain hundreds of cells.

The cells reproduced at a rapid rate.

Spectrum Reading Grade 6

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