Although it weighs just 0.24 oz. (7 g.) and has an average diameter of only 0.94 in (24 mm), the human eye is a biological camera whose complexity and capacities surpass those of the most advanced optical apparatuses. This highly evolved optical system includes two lenses and a pupil that are responsible for deflecting a precise quantity of light rays toward the retina, where more than 130 million photoreceptors convert the light into neural signals that can be interpreted by the brain.
The lacrimal glands, located above each eye, are constantly secreting tears. Everytime the eyelid blinks, it causes this lacrimal liquid to flow over the surface. This keeps the eye moist and free of dust and microbes. They eyelids and eyelashes also play a protective role.
The eye, recessed within a bony socket, is a hollow body filled with a gelatinous substance called the vitreous body. It is covered with several layers of tunics that form the coat of the eyeball: the retina, the choroid and the sclera. At the front of the eye, the sclera becomes perfectly transparent to form the cornea.
Light enters the eye through the cornea, which is the principal ocular lens. It then travels through the opening in the pupil. Behind the pupil is the crystalline lens, which converges light rays toward the retina.
Light rays that reach the retina pass through several layers of cells before reaching the photoreceptor cells, the only cells that have pigments capable of transforming light into electrical impulses. These impulses are transmitted by intermediary neurons to the optic nerve, which carries the information to the brain.
The retina contains two types of photoreceptor cells: cones and rods. There are many more rods (125 million) than cones; although they do not perceive colors, rods are very sensitive to contrasts in light. On the other hand, cones (6 million) perceive colors perfectly.
Human beings have remarkable visual ability; in fact our sense of sight is vastly superior to our other senses. The perception of shapes, distances, colors and movements in our environment is a complex process that uses a chain of optical and nervous components, from the cornea to the cortex.
However, the precision of this optical system makes it particularly fragile: the slightest imperfection in the shape of the eyeball or the curve of the cornea leads to an imbalance for which the crystalline lens cannot always compensate. In these cases, because the image is not focused on the retina but in front or behind it, vision is blurry.
Myopia is defect in which the image of distant objects is formed in front of the retina. This situation is corrected with a concave lens, which pushes the point of convergence of the light rays farther back in the eye.
In hypermetropia, by contrast, the image is formed behind the retina. To correct this problem, a convex lens is used to bring the point of convergence forward in the eye. Astigmatism is a defect in the curve of the cornea or the crystalline lens preventing homogeneous convergence of light rays. An asymmetrical lens can correct this problem.