The Eye Is a Natural Optical Tool

KEY CONCEPT The eye is a natural optical tool.

BEFORE, you learned NOW, you will learn •Mirrors and lenses focus light • How the eye depends on to form images natural lenses •Mirrors and lenses can alter • How artificial lenses can be images in useful ways used to correct vision problems

VOCABULARY EXPLORE Focusing Vision cornea p. 607 How does the eye focus an image? pupil p. 607 retina p. 607 PROCEDURE

1 Position yourself so you can see an object about 6 meters (20 feet) away.

2 Close one eye, hold up your index finger, and bring it as close to your open eye as you can while keeping the finger clearly in focus.

3 Keeping your finger in place, look just to the side at the more distant object and focus your eye on it.

4 Without looking away from the more distant object, observe your finger.

WHAT DO YOU THINK? • How does the nearby object look when you are focusing on something distant? • What might be happening in your eye to cause this change in the nearby object?

The eye gathers and focuses light. The eyes of human beings and many other animals are natural optical tools that process visible light. Eyes transmit light, refract light, and respond to different wavelengths of light. Eyes contain natural lenses that focus images of objects. Eyes convert the energy of light waves into signals that can be sent to the brain. The brain interprets these signals as shape, brightness, and color. Altogether, these processes make vision possible. In this section, you will learn how the eye works. You will also learn how artificial lenses can be used to improve vision.

606 Unit 4: Waves, Sound, and Light How Light Travels Through the Human Eye 1 Light enters the eye through the cornea (KAWR-nee-uh), a transpar- entmembrane that covers the eye. The cornea acts as a convex lens and does most of the refracting in the eye. 2 The light then continues through the pupil, a circular opening that controls how much light enters the eye. The pupil is surrounded by the iris, which opens and closes to change the size of the pupil.

3 Next the light passes through the part of the eye called the lens. reading tip The lens is convex on both sides. It refracts light to make fine The word lens can refer both to an artificial optical adjustments for near and far objects. Unlike the cornea, the lens is tool and to a specific part attached to tiny muscles that contract and relax to control the of the eye. amount of refraction that occurs and to move the focal point. 4 The light passes through the clear center of the eye and strikes the retina (REHT-uhn-uh). The retina contains specialized cells that respond to light. Some of these cells send signals through the optic nerve to the brain. The brain interprets these signals as images.

How the Human Eye Forms an Image

The cornea and lens together focus a reduced, inverted image on the retina.

1 Light from an object is refracted iris by the cornea. 4 An image is focused on the retina.

2 The light passes through the iris and the pupil.

3 optic The lens refracts nerve the light more.

What part of the illustration explains why the image is focused upside down? Explain your answer.

Chapter 18: Light and Optics 607 How the Eye Forms Images For you to see an object clearly, your eye must focus an image of the object on your retina. The light reflected from each particular spot on the object must converge on a matching point on your retina. Many COMBINATION NOTES such points make up an image of an entire object. Because the light rays Make a chart showing how pass through the lens’s focal point, the image is upside down. The brain light interacts with different parts of the eye. interprets this upside down image as an object that is right-side up. For a complete image to be formed in the eye and communicated to the brain, more than the lens and the cornea are needed. The retina also plays an important role. The retina contains specialized cells that detect brightness and color and other qualities of light. Rod Cells Rod cells distinguish between white and black and shades of gray. Rods respond to faint light, so they help with night vision. Cone Cells Cone cells respond to different wavelengths of light, so they detect color. There are three types of cones, one for each of the colors red, blue, and green. Cones respond to other colors with combinations of these three, as the screen of a color monitor does. The brain interprets these combinations as the entire color spectrum.

check your reading Which type of cell in the retina detects color?

Vision SKILL FOCUS How does distance affect vision? Observing PROCEDURE

1 Arrange the materials as shown so that the lamp shines through the lens onto the plate. The lens should be about 2 a meter from the lamp. 3 MATERIALS 2 Adjust the distance between the plate and the lens until you see a focused • convex lens image of the bulb on the plate. Measure this distance. •index card • modeling clay 3 Move the lens until it is about a meter and a half from the lamp. • white paper Adjust the plate once again to get a focused image, then plate measure the distance between the plate and the lens. • lamp TIME WHAT DO YOU THINK? 10 minutes • How does the distance needed between the plate and the lens change when the lamp is farther from the lens? •How is what happens in the eye different from what you did to refocus the image?

CHALLENGE How could you change the model to make it more like what happens in the eye?

608 Unit 4: Waves, Sound, and Light Corrective lenses can improve vision. What happens when the image formed by the lens of the eye does not fall exactly on the retina? The result is that the image appears blurry. This can occur either because of the shape of the eye or because of how the lens works. Artificial lenses can be used to correct this problem.

Corrective Lenses A person who is nearsighted cannot see objects clearly unless they are near. Nearsightedness occurs when the lens of the eye focuses the image in front of the retina. The farther away the object is, the farther in front of the retina the image forms. This problem can be corrected with glasses made with concave lenses. The concave lenses spread out the rays of light before they enter the eye. The point at which the rays meet then falls on the retina.

nearsighted eye concave lens

1 image in front of retina 2 image at retina

Objects are clearer to a farsighted person when the objects are farther away. Farsightedness occurs when the lens of the eye focuses an object’s image behind the retina. This condition can result from

aging, which may make the lens less flexible. The closer the object is, reading tip the farther behind the retina the image forms. Farsightedness can be Nearsighted people can see corrected with glasses made from convex lenses. The convex lenses objects near to them best. Farsighted people can see bend the light rays inward before they enter the eye. The point at objects better when the which the rays meet then falls on the retina. objects are farther away.

farsighted eye convex lens

1 image behind retina 2 image at retina

check your reading |What kind of lens is used for correcting nearsightedness?

Chapter 18: Light and Optics 609 Surgery and Contact Lenses Wearing glasses is an effective way to correct vision. It is also possible to change the shape of the cornea to make the eye refract properly. The cornea is responsible for two-thirds of the refraction that takes place inside the eye. As you know, the eye’s lens changes shape to focus an image, but the shape of the cornea does not ordinarily change. However, using advanced surgical technology, doctors can change the shape of the cornea. By doing this, they change the way light rays focus in the eye so that the image lines up with the retina. To correct for nearsightedness, surgeons remove tissue from the center of the cornea. This flattens the cornea and makes it less convex so that it will refract less. To correct for farsightedness, surgeons remove tissue from around the edges of the cornea. This increases the cornea’s curvature to make it refract more. Surgery changes the shape of the cornea permanently and can eliminate the need for eyeglasses. Contact lenses fit directly onto the cornea, changing Contact lenses also correct vision by changing the way the cornea the way light is refracted as it enters a person’s eye. refracts light. Contact lenses are corrective lenses that fit directly onto the cornea. The lenses actually float on a thin layer of tears. The moisture, the contact lens, and the cornea all function together. The lens of the eye then focuses the light further. Because the change is temporary, contacts, like eyeglasses, can be adapted to new changes in the eye.

check your reading What are two ways of changing the way the cornea refracts light to correct vision?

KEY CONCEPTS CRITICAL THINKING CHALLENGE 1. Where are images focused in 4. Make a Model Draw a dia- 6. Apply A person alternates an eye with perfect vision? gram to answer the following between wearing glasses and 2. What causes people with question: How does a convex wearing contact lenses to cor- nearsightedness to see blurry lens affect the way a near- rect farsightedness. Are the images of objects at a sighted eye focuses an image? contact lenses more or less distance? 5. Analyze What distance would convex than the lenses of the glasses? Explain the reasoning 3. What kind of lens is used for an eye doctor need to measure behind your response. correcting farsightedness? to correct a problem with near- Why? sightedness or farsightedness?

610 Unit 4: Waves, Sound, and Light