3. Eye Problems and their Correction
People with perfect vision can usually see clearly objects from 10 cm to an infinite length away. However, many people have eye defects that for one reason or another do not allow them this vast range of vision. Out of your very own acquaintances, there are probably a great deal of people with glasses of some sort. Thanks to Benjamin Franklin and many others, it is possible for these people to see clearly with the aid of a simple optic device: the lens.
The two main kinds of eye problems are myopia, or shortsightedness, and hyperopia, or hypermetropia, known as farsightedness. Myopia is a condition that arises when the eyeball is effectively too long. Parallel light rays from far objects are bent by the lens so that they are not able to converge on the retina Instead they meet in front, or fall short of the retina, resulting in the more colloquial name for this condition. Shortsighted people are only able to see things that are fairly close, with farther objects appearing blurry to them. A diagram of a myopic eye is shown below (the eye in this case has been simplified so that there is effectively only the lens):
In a hypermetropic eye, the opposite happens. The eyeball is considered to be too short. The lens is not able to make light waves from closer objects converge enough to meet on the retina. Instead, if they were able too, these light rays would meet behind the eye. This condition only allows objects that are farther to be seen clearly. A diagram of a hyperopic eye is given below, showing where the light rays would meet if they were able to continue:
In a myopic eye, light rays basically converge too readily. The lens required to correct this condition therefore must have a negative power, or convergence capacity. Such a lens is necessarily a diverging (or concave) lens. This makes light beams from farther away diverge a little before entering the eye, meaning that they meet farther from the lens than they normally would. Here is a diagram of myopic sight being corrected by a diverging lens:
Once again, for a hyperopic eye the opposite is true. Light rays need to be able to converge sooner. A lens that has a positive convergence capability, or power, is put between light waves coming from a close object and the eye. A converging (or convex) lens is used to do some of the work for the eye, by making the light rays converge a little before they even enter the eye. A diagram for the correction of the hypermetropic condition is shown below:
A third eye condition, known as presbyopia arises as people grow older. Presbyopia is the stiffening of the eye lens, so that it is less capable of accomodation (being adjusted) by the ciliary muscles. What this means is that the eye is still capable of seeing things that are at a distance from the eye, as the ciliary muscles do not need to do work, but that closer things are harder to see, as the lens can no longer be squeezed into a higher power. The condition of a higher lens power is then simply achieved by once again placing a convex (converging) lens in front of the eye.