The (human) eye is a natural optical instrument that has developed and evolved over the years. The adult eye can be described physically as a white, spongy, water-filled sphere, approximately 25 mm (one inch) in diameter (Fig. 2.1). On the front surface of this sphere there is a circular transparent spot about 10 mm in diameter. The surface of the sphere bulges outward slightly in this area, forming what is known as the cornea. Because the eye is filled with a water-like fluid, it is the first surface of the cornea that produces most of the optical lens power of the eye. In terms of diopters, the typical unaccommodated eye (viewing an object at infinity) has about 57 diopters of total power. Of this, approximately 43 diopters are provided by the cornea. Typically, as shown in Fig. 2.1, the cornea is 0.6 mm thick, and it is followed by an approximately 3-mm-thick layer of a water-like fluid called the aqueous. Light passing through the cornea and aqueous then encounters the eyelens, which is suspended within a muscular structure that is capable of changing the lens shape to provide focus on near objects, a process called accommodation. The iris of the eye, which gives it its color, is located just in front of the eyelens. The circular opening in the iris is adjustable over a useful range of from 1- to 7-mm diameter. The black spot that we see at the center of the iris is actually the front surface of the eyelens.
For purposes of analysis in the sections and chapters to follow, we have assigned fixed values to the variable features of the eye. For most examples it will be assumed that the eye is not accommodated, that is, its focus is set to view objects at infinity. For a few special cases, which will be noted, we will assume that the eye is fully accommodated to view objects at a distance of 254 mm (10 inches), the accepted near point of vision for the typical eye. Note in Fig. 2.1 that the eyelens assumes a nearly symmetrical shape when the eye is focused on objects at the near point.
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