How Things Work: Vision problems and glasses
Glasses come in all shapes and sizes, and so do the eyes using them. Despite their diversity, all human eyes work in relatively the same fashion; so what exactly is happening inside a set of eyes that need glasses?
To answer this question, it’s necessary to explain how the eye views an image. As light reaches the eye, it first passes through the cornea — the clear tissue at the front of the eye — and enters the eye through the pupil. Next, it passes through the lens, which is behind the pupil, and finally reaches the retina at the back of the eye.
In a perfectly-functioning eye, the lens and the cornea refract, bending light rays and focusing them on the retina. Due to this refraction, the image appears inverted on the retina, so it must be adjusted by the brain.
Light from images is focused on the center of the retina. There are a number of vision problems that prevent this, however, and they make wearing glasses necessary. These vision problems — known as refractive errors — are myopia, hyperopia, astigmatisms, and presbyopia.
Myopia, or nearsightedness, means the eyeball is longer than normal so the retina is too far back. In this case, the eye is too long for the lens and cornea to properly focus. As a result, instead of reaching the retina, light from images is focused in front of the retina. According to the American Optometric Association, nearly 30 percent of the U.S. population is affected by myopia, and growing evidence shows that the condition is influenced by visual stress.
Nearsightedness can be treated with glasses or contacts with convex-shaped lenses. Convex lenses cause light to scatter, and it must be converged again by the lens of the eye. This shifts the focal point of the eye back, so that the image is focused on the retina instead of in front of it.
On the other hand, people with hyperopia, or farsightedness, have shorter eyeballs; their retinas are too far forward, resulting in images being focused behind the cornea. Farsighted individuals can view distant objects with relative ease, but struggle to see nearby objects. Hyperopia can be corrected by glasses or contacts with concave lenses, which cause light to be refracted toward the center. As a result, the focal point of the eye’s lens is pushed forward so the image is focused on the retina rather than behind it.
Myopia and hyperopia may be accompanied by astigmatisms, which occur when an individual’s cornea has an irregular shape, causing light from images to refract in different directions rather than at one focal point on the retina. As a result, astigmatisms usually blur images.
They can be treated with eyeglasses that have spherical lenses, which compensate for the irregular shape of the cornea and prevent light from being refracted in different directions. They can be also be corrected with orthokeratology, which involves wearing specially-designed, rigid contact lenses that slowly adjust the shape of the cornea so it bends light correctly.
Presbyopia is caused by age rather than eye structure. In order to properly focus light, the lens of the eye needs to be able to widen and compress. However, around age 40, the lens of the eye begins to harden and lose its flexibility. As a result, the eye has difficulty bringing both near and far images into focus.
Presbyopia can be corrected with bifocal lenses, which can focus on distant images throughout the lens and on near images at the bottom of the lens. It can also be addressed with specially designed contact lenses that either have a reading and distance prescription for each eye, or have a reading prescription for one eye and a distance prescription for the other.
Myopia, hyperopia, presbyopia, and astigmatisms can also be treated with LASIK eye surgery. The principle behind LASIK is to apply a laser beam to the cornea to remove enough of the corneal tissue to reshape the eye as needed, which involves cutting open and pulsating a laser on the cornea until enough of the tissue is removed.