When it is relaxed, a typical, "normal" eye is focused at infinity. Light coming from a nearby object is more divergent as it enters the eye and must be bent more. The lens provides this additional refraction as the ciliary muscles pull on it. The closest distance at which an object can be clearly seen or focused is called the near point. For a typical, "normal" eye the near point is about 25 cm. Likewise, the far point is the maximum distance at which an object can be clearly seen or focused. As with height, hair color, IQ, or lung capacity, there is enormous variation in the near point or far point from person to person. A "normal" eye is more often found in textbooks than in people.
Nearsightedness or myopia refers to a condition
in which nearby objects can be focused or seen clearly but distant
objects can not be clearly focused. The far point is not infinity.
There is too much bending-or too much "power"-in the
cornea and the lens. Distant objects are focused in front of
the retina. A diverging lens placed in front of
the eye will make parallel rays of light from a far-distant object
divergent as if they were coming from a nearby object. The myopic
eye will then be able to correctly focus these rays on the retina.
This is illustrated in Figure 19.9.
Farsightedness or hyperopia refers to a condition in which
distant objects can be focused or seen clearly but nearby objects
can not be clearly focused. The near point may be too great to
allow comfortable reading (hence the joke about "needing
to get new glasses or longer arms"). There is too little
bending-or too little "power"-in the cornea and the
lens. Nearby objects are focused behind the retina. A converging
lens placed in front of the eye will make divergent rays of light
from a nearby object less divergent or even parallel as if they
were coming from a distant object. The hyperopic eye will then
be able to correctly focus these rays on the retina. This is illustrated
in Figure 19.10. We will discuss these corrective lenses more
thoroughly in the following section.