a. Retinal Image Formation. Four basic processes are involved in the
formation of an image on the retina: refraction of light rays, accommodation of the lens,
constriction of the pupil, and convergence of the eyes. Accommodation and pupil size
are caused by intrinsic eye muscles (muscles inside the eyeball). Extrinsic eye muscles
(1) Refraction of light rays. When light rays traveling through a transparent
medium (such as air) and pass into a second transparent medium with a different
density (such as water), they bend at the surface of the two media. This is refraction.
The eye has four such media of refraction: cornea, aqueous humor, lens, and vitreous
humor. Light rays entering the eye from the air are refracted at the following points:
(a) The anterior surface of the cornea as they pass from the lighter air
into the denser cornea.
(b) The posterior surface of the cornea as they pass into the less
dense aqueous humor.
(c) The anterior surface of the lens as they pass from the aqueous
humor into the denser lens.
(d) The posterior surface of the lens as they pass from the lens into the
less dense vitreous humor.
(2) Degree of refraction. The degree of refraction that takes place at each
surface in the eye is very precise. Light rays from an object 20 feet away must bend to
fall on the central field, where vision is the sharpest. Light rays from objects that are
near are divergent rather than parallel. As a result, these light rays must be refracted
toward each other to a greater extent. The lens of the eye makes this change.
(3) Accommodation of the lens. The lens of the eye is biconvex; that is, it
curves outward on both sides. If the surface of a lens curves outward, as in a convex
lens, the lens will refract incoming rays toward each other so they eventually intersect.
The greater the curve, the more acutely it bends the rays toward each other.
Conversely, when the surface of a lens curves inward, as in a concave lens, the rays
bend away from each other. The lens of the eye has the unique ability to change the
focusing power of the eye by becoming moderately curved at one moment and greatly
curved the next. When the eye is focusing on a close object, the lens curves greatly in
order to bend the rays toward the central fovea. This increase in the curvature of the
lens is called accommodation. In near vision, muscles cause the lens to become
shortened, thickened, and bulged. In far vision, muscles cause the lens to flatten.