For our purposes, the normal human eye consists of the following parts, listed in the order in which they are encountered by the light entering the eye. (The numbers in parentheses are the indices of refraction of the various portions of the eye.) See Fig. 3.1 below.
- Cornea: The cornea is the transparent portion of a mostly white and opaque shell which encases the eye, called the sclera. The cornea is responsible for the majority of the “focusing power” of the eye (that is, the cornea is responsible for the majority of the bending, and thus the focussing, of the light by refraction). (n = 1.376)
- Aqueous Humor: The aqueous humor is a watery fluid that fills the chamber between the cornea and the lens of the eye. (n = 1.336)
- Lens: The aperture size of the lens is controlled by the iris, which is a flexible diaphragm. The opening in the iris in front of the lens is called the pupil. The lens itself is layered much like an onion, and is about 9 mm in diameter and 4 mm thick. The lens is primarily responsible for the “fine tuning” of the eye’s vision by means of a process called accommodation. Accommodation is achieved by means of the ciliary muscle, which is attached to the edge of the lens. When the ciliary muscle is relaxed, the lens tends to be flattened, and the normal eye is focused on distant objects. But when the ciliary muscle contracts, it allows the front surface (toward the cornea) to bulge slightly, making it more curved. This increased curvature (decreased radius of curvature) reduces the focal length of the lens, thereby enabling the eye to focus on closer objects. When the ciliary muscle is fully contracted, the focal length of the lens is reduced to its smallest value, and the eye is focused on objects at the smallest distance possible for clear vision. This closest object position for clear vision is called the near point, and corresponds to a distance that we shall denote Dmin. For a normal eye, the near point is about 25 cm from the eye. (n varies from a value of about 1.406 at its core to about 1.386 at its edges.)
- Vitreous Humor: The vitreous humor is a gelatinous substance that fills the large chamber between the eye lens and the retina. (n = 1.337)
- Retina: The retina is a delicate layer of cells about 0.1 mm to 0.5 mm thick that covers about 65% of the interior surface of the large chamber behind the lens. The retina contains about 125 to 130 million cells that are responsible for converting the energy carried by the EM radiation (light) into electrical nerve impulses which can then be transported by the optic nerve to the brain, where the signals are processed and interpreted as an image. There are two types of such photoreceptor cells: rods, which are good at detecting low levels of light, and cones, which are good at detecting brighter light and distinguishing colors. When you look directly at an object, the image tends to fall in the central region of the retina behind the lens called the fovea, which contains predominantly cones.