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.
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?
562 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.