19.2 The Human Eye

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.

Figure 19.9 A nearsighted or myopic eye can clearly focus on nearby objects but bends light from infinity too greatly, bringing it to a focus in front of the retina. A diverging lens in front of the eye can make the light appear to come from nearby so the eye can then clearly focus it.

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.

Figure 19.10 A farsighted or hyperopic eye can clearly focus on distant objects but lacks the power to bend light from nearby objects enough to focus the light on the retina; it is focused behind the retina. A converging lens in front of the eye can make the light appear to come from far away so the eye can then clearly focus it.

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