5.60. Vision Problems and Corrective Lenses http://www.ck12.org
Nearsightedness
Nearsightedness, or myopia, is the condition in which nearby objects are seen clearly, but distant objects appear
blurry. TheFigure5.141 shows how it occurs. The eyeball is longer (from front to back) than normal. This causes
images to be focused in front of the retina instead of on the retina. Myopia can be corrected with concave lenses.
The lenses focus images farther back in the eye, so they fall on the retina instead of in front of it.
FIGURE 5.141
Q:Sometimes squinting the eyes can help someone see more clearly. Why do you think this works?
A:Squinting may improve focus by slightly changing the shape of the eyes. When you squint, you tighten muscles
around the eyes, putting pressure on the eyeballs.
Farsightedness
Farsightedness, or hyperopia, is the condition in which distant objects are seen clearly, but nearby objects appear
blurry. It occurs when the eyeball is shorter than normal (seeFigure5.142). This causes images to be focused in a
spot that would fall behind the retina (if light could pass through the retina). Hyperopia can be corrected with convex
lenses. The lenses focus images farther forward in the eye, so they fall on the retina instead of behind it.
FIGURE 5.142
Q:Joey has hyperopia. When is he more likely to need his glasses: when he reads a book or when he watches TV?
A:With hyperopia, Joey is farsighted. He can probably see the TV more clearly than the words in a book because
the TV is farther away. Therefore, he is more likely to need his glasses when he reads than when he watches TV.
Summary
- Many people have vision problems. Often, the problem is due to the shape of the eyes and how they focus
light. Two of the most common vision problems are nearsightedness and farsightedness.