How the Eye Sees
When a person looks at an object, light rays bouncing off that object hit the cornea, which bends them toward the center of the retina by a process known as refraction. These bent light rays then go through the pupil and are adjusted by the lens. If the eye is focusing properly, the rays then fall onto the retina (mostly on the macula) as a clear, upside-down image. This image is captured by the photoreceptors and then transferred via the nerve fiber layer of the retina to the optic nerve, which connects to the brain, where the image is interpreted and inverted so the world appears right-side-up.
When the eye does not properly focus light rays from the image, the result is a refractive error. There are two types of refractive errors:
- Spherical aberration. Spherical aberration occurs when the focused image falls either in front of the retina (near-sightedness, or myopia) or “behind” the retina (far-sightedness, or hyperopia).
- Astigmatism. Astigmatism occurs when light waves interfere with each other as their distance from the central axis increases so that the image becomes blurry anywhere except in the middle. Astigmatism is usually caused by an irregularly shaped cornea. It can occur alone or in conjunction with myopia or hyperopia.
Myopia, hyperopia, and astigmatism can all usually be corrected with eyeglasses, contact lenses, or refractive surgery such as LASIK.
Besides refractive errors, most people develop presbyopia at around age 40. This disease occurs when the lens of the eye can no longer focus on near images because of age-related stiffness. For people who never wore glasses, reading glasses are usually necessary starting at around age 40. For those who already wear glasses, bifocals or another form of dual correction is often needed. Presbyopia is an irreversible condition.