Thanks to the ingenuity of people of the past, eye glasses have a long and profitable history. The first ones appeared about the time of Christ, in China. We are told Nero used an emerald to aid his vision. Today exact correction is possible through glass grinding or plastic lens creation. All this is needed because of common eye problems we may have inherited or acquired.
Sight is impossible until light enters our eyeballs and changes to electrical impulses at the retina in the back of the eye. If any of this process is hindered, then vision is either impaired, or stopped. Light enters the eyeball by passing through the cornea or the outer convex disk that is filled with a liquid called the aqueous humor. The light then passes through the pupil, a hole whose diameter changes as the iris opens or closes. Next the light is refracted by the lens. This clear disk changes shape automatically by means of muscles so that it can keep images in focus. Refraction means the light is bent out or in so that it focuses on the retina, the back of the eyeball. At the retina, light is converted into electrical impulses that are then sent via the optic nerve to the brain.
Let’s focus on the focus mechanism. Whether or not an image is in focus depends on the shape of the cornea, the ability of the lens to change shape, and the length of the eyeball. Though the vision mechanism is a marvelous design, it does not always work perfectly for everyone. For example, if the eyeball is too long, light focuses in front of the retina, causing blurry vision of distant objects. This is called myopia or nearsightedness. Such a person will be able to see close objects such as a book clearly, but distant objects such as a street sign may be blurry. Until recently, the only way to correct this was by placing a concave lens in front of the eye thus bending the light out so that it focuses correctly on the retina.
If the eyeball is too short, the light has not focused by the time it reaches the retina. This is called hyperopia or farsightedness. Such a person will be able to see distant objects but will find close objects blurry. A convex lens helps to bend the light in so that it will focus sooner, at the retina.
Some people do not see well for either focal distance. They may need bifocals, the upper part to correct for farsightedness, and the lower part to correct for nearsightedness. One cause of this is a stiff lens, often evident in older people. The lens cannot change shape as it used to and thus objects at both focal lengths are blurry.
Astigmatism is caused when either the cornea or lens shape is distorted. This causes multiple images on the retina giving blurry vision at all distances. It can be corrected with specially designed lenses that correct the irregularities.
In recent years, thanks to the invention and application of laser light, all of these eye problems can be reduced or eliminated by Lasik surgery.