The eye, like a camera, focuses light rays to an inverted image on the retina. Perhaps surprisingly, most of the refractive power of the eye is due to the cornea, not the lens. The cornea is a sharply curved, spherical surface, and you learned in Chapter 34 that images are formed by refraction at a spherical surface. The rather large difference between the index of refraction of air and that of the aqueous humor causes a significant refraction of light rays at the cornea. In contrast, there is much less difference between the indices of the lens and its surrounding fluid, so refraction at the lens surfaces is weak. The lens fine-tunes the eye’s focus, but the air-cornea boundary is responsible for the majority of the refraction.

You can recognize the power of the cornea if you open your eyes underwater. Everything is very blurry! When light enters the cornea through water, rather than through air, there’s almost no difference in the indices of refraction at the surface. Light rays pass through the cornea with almost no refraction, so what little focusing ability you have while underwater is due to the lens alone.

A camera focuses by moving the lens. The eye focuses by changing the focal length of the lens, a feat it accomplishes by using the ciliary muscles to change the curvature of the lens surface. The ciliary muscles are relaxed when you look at a distant scene. Thus the lens surface is relatively flat and the lens has its longest focal length. As you shift your gaze to a nearby object, the ciliary muscles contract and cause the lens to bulge. This process, called accommodation, decreases the lens’s radius of curvature and thus decreases its focal length.

The farthest distance at which a relaxed eye can focus is called the eye’s far point (FP). The far point of a normal eye is infinity; that is, the relaxed eye can focus on objects extremely far away. The closest distance at which an eye can focus, using maximum accommodation, is the eye’s near point (NP). (Objects can be seen closer than the near point, but they’re not sharply focused on the retina.) A near point of 25 cm—typical of young adults—is considered to be normal vision. Both situations are shown in FIGURE 35.8.

FIGURE 35.8 Normal vision of far and near objects.