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KEY CONCEPT
The eye is a natural
optical tool.
BEFORE, you learned
NOW, you will learn
• Mirrors and lenses focus light
to form images
• Mirrors and lenses can alter
images in useful ways
• How the eye depends on
natural lenses
• How artificial lenses can be
used to correct vision problems
VOCABULARY
EXPLORE Focusing Vision
cornea p. 607
pupil p. 607
retina p. 607
How does the eye focus an image?
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
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How Light Travels Through the Human Eye
1 Light enters the eye through the cornea (KAWR-nee-uh), a transparent membrane 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.
The lens is convex on both sides. It refracts light to make fine
adjustments for near and far objects. Unlike the cornea, the lens is
attached to tiny muscles that contract and relax to control the
amount of refraction that occurs and to move the focal point.
4
reading tip
The word lens can refer
both to an artificial optical
tool and to a specific part
of the eye.
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
by the cornea.
iris
4
An image is
focused on
the retina.
2
The light passes
through the iris
and the pupil.
optic
nerve
3
The lens refracts
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
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COMBINATION NOTES
Make a chart showing how
light interacts with different
parts of the eye.
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
such points make up an image of an entire object. Because the light rays
pass through the lens’s focal point, the image is upside down. The brain
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
How does distance affect vision?
SKILL FOCUS
Observing
PROCEDURE
1
Arrange the materials as shown so that the lamp shines through the lens
2
onto the plate. The lens should be about a meter from the lamp.
3
2 Adjust the distance between the plate and the lens until you see a focused
image of the bulb on the plate. Measure this distance.
3 Move the lens until it is about a meter and a half from the lamp.
Adjust the plate once again to get a focused image, then
measure the distance between the plate and the lens.
WHAT DO YOU THINK?
• 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
MATERIALS
•
•
•
•
convex lens
index card
modeling clay
white paper
plate
• lamp
TIME
10 minutes
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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
1
image in front of retina
concave
lens
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,
the farther behind the retina the image forms. Farsightedness can be
corrected with glasses made from convex lenses. The convex lenses
bend the light rays inward before they enter the eye. The point at
which the rays meet then falls on the retina.
farsighted eye
1
image behind retina
check your reading
reading tip
Nearsighted people can see
objects near to them best.
Farsighted people can see
objects better when the
objects are farther away.
convex
lens
2
image at retina
What kind of lens is used for correcting nearsightedness?
|
Chapter 18: Light and Optics 609
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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.
Contact lenses fit directly
onto the cornea, changing
the way light is refracted
as it enters a person’s eye.
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 also correct vision by changing the way the cornea
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
1. Where are images focused in
an eye with perfect vision?
4. Make a Model Draw a diagram to answer the following
question: How does a convex
lens affect the way a nearsighted eye focuses an image?
2. What causes people with
nearsightedness to see blurry
images of objects at a
distance?
3. What kind of lens is used for
correcting farsightedness?
Why?
610 Unit 4: Waves, Sound, and Light
5. Analyze What distance would
an eye doctor need to measure
to correct a problem with nearsightedness or farsightedness?
CHALLENGE
6. Apply A person alternates
between wearing glasses and
wearing contact lenses to correct farsightedness. Are the
contact lenses more or less
convex than the lenses of the
glasses? Explain the reasoning
behind your response.