Everyone knows that eyeglasses allow people to see clearly. However, few really understand how glasses actually work. Vision occurs when light enters the eye and lands on the retina, a light-sensitive membrane located at the back of the eye.
The retina then transmits the light message to the brain through the optic nerve. The brain interprets the message and transforms it into an image. This is the sense of sight.
Problems with clear eyesight can result when light entering the eye doesn’t bend correctly. This bending of light is called refraction. You can see refraction at work when you insert a straight object into a glass of water.
The object will appear to be curved, even though it is straight. This refraction happens when the light goes from one level of density to another. In the example of the straight object, it is going from air into water.
The same thing happens when the light goes from the air into the eye. It gets bent. If the eye adjusts the refraction perfectly, the light lands on just the right spot on the retina, and the result is a perfectly clear image. If it does not, then glasses are required to correct the faulty refraction.
If the light entering the eye falls short of the retina’s sweet spot, it will cause a blurry image of objects that are further away. This is called nearsightedness. The medical term is myopia.
If the light lands behind the retina, farsightedness, or hyperopia, will result. This means that the person can see well at a distance, but not close up. There are varying degrees of both of these conditions.
As a person gets older, natural age-related changes occur in the eye’s ability to focus on close-up objects. This condition is called presbyopia and tends to occur somewhere in the early to mid-forties.
It can be corrected with special lenses called bifocals. These have an obvious area along the lenses’ bottoms that looks like a little window.
It’s actually a different focal point for close-up vision. Bifocals can be hard to adjust to, and many people find them unattractive. In more recent years, bifocals have been replaced with progressive lenses.
These have no obvious areas of demarcation and allow a smooth flow from several different focal points.
Another common visual problem is called astigmatism. This is caused by imperfections in the curvature of the cornea or front part of the eye.
This imperfection causes additional problems with refraction that typically result in blurry, fuzzy or distorted vision. There are varying degrees of astigmatism, too.
How to Read an Eyeglass Prescription
This document may look odd, with strange terms and numerical notations, but it’s really not hard to understand. Typical eyeglass prescriptions are talking about concave, convex, and cylindrical lenses.
A myopic lens will always be thicker at the edges and thinner in the middle. One for hyperopia will be the opposite, that is, thinner at the edges and thicker in the middle.
Modern lens materials have done a lot to eliminate overly thick, heavy lenses, but they will still have the same basic formation. When reading an eyeglass prescription, you will notice some terms you may not recognize.
OD is a Latin abbreviation meaning right eye. OS means the left. It’s possible to need correction in only one eye.
In that case, the normal eye is said to be plano and will have a number of 0.0. The space may also be left blank. Plano is also called 20/20 vision.
Then you will see words like sphere, cylinder, and axis.
The sphere means the power of the lens required to correct the faulty refraction. Negative numbers refer to myopia. Positive ones refer to hyperopia. The CYL, or cylinder, is a measure of how much astigmatism there is and where it’s located on the eyeball’s curvature.
The corrective lens will be made to compensate for these areas. This is called the axis. Blank areas under the CYL mean that there is no astigmatism to correct. However, some degree of astigmatism is very common.
Lower negative and positive numbers refer to lower degrees of refraction defects. For example, a -1.00 is only mild myopia, but someone with a -8.00 sees only blurry colors and vague shapes at any kind of distance at all.
Prism refers to correcting problems with eye alignment. The up and down notations are part of that. The add section means that the lens needs to correct both distance and close-up vision. In other words, someone could be both a sphere -7.00 and an add +2.00.
The +2.00 is going to be toward the bottom part of the lens so the person can read easily. The -7.00 will be towards the top so the person can see far objects clearly.
How Long is an Eye Prescription Good For?
There is no simple answer to this question. Children and adolescents tend to have rather rapidly changing prescriptions because their eyes are growing and changing along with the rest of them.
As a person nears their late teens, these changes tend to slow down considerably and may stop altogether. An adult may be fine with the same prescription for many years, at least until age-related presbyopia appears.
However, even people with perfect vision will still eventually experience presbyopia.
Regular eye exams will help catch subtle changes in visual acuity early.
How to Clean Eyeglasses
Glasses will naturally collect dust and skin oils as you wear them. This can cause annoying dots and smudges in your field of vision. Simply wiping them will only smear the skin oils even more.
You need to use a cleaner that is safe for the lens and also breaks up skin oils. There are commercial cleaners that you can buy, but you can also use plain dish soap. Dawn or some generic similar brand is good. Follow this procedure:
1. Wash your hands.
2. Rinse the lenses under clear, warm water to remove any particles. This is important. If you rub your lenses with particles of dirt or sand still on them, it could scratch them.
3. Put a few drops of dish soap on your hands and rub them together. Use the suds to clean each side of each lens. Rinse them thoroughly.
4. Use a soft, lint-free cloth to dry the lenses and frames. An old, clean cloth diaper or cotton washcloth is good. Never, ever use paper towels or tissue. These are made from wood and can scratch your lenses.
Where to Donate Old Eyeglasses
Just because you no longer need your glasses doesn’t mean they are useless. Donate them! Many optical shops have donation buckets. Wal-Mart has them, too.
Goodwill often accepts them. So do many charities and churches. Ask your eye care professional for the nearest place to donate your old glasses.
If they don’t take them, they should be able to direct you to someplace that does. Lots of people could get good use of your old glasses. They’re no good to you anymore. Donate!
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