Eyeglasses are basically lenses or prisms worn in front of the eyes to correct various vision errors. This is usually in the form of a pair of glass or hard plastic lenses fitted into a metal or plastic frame that sits on the bridge of the nose. Although hard plastic lenses can scratch more easily, they are now more common than glass because of the increased benefits of reduced weight and safety from breakage. Glasses are held in place by pads on either side of the nose, and by arms, bows, or temples that grip the side of the head or hook around behind the ears.

Other types of glasses include those held in place by spring pressure on the nose, usually called pince-nez, single lenses called monocles that are used to correct vision in one eye only and are held in place by wedging the lens into the orbit of the eye, and reading glasses called lorgnettes that have a handle one side instead of arms.

Eyeglass lenses are ground in many different shapes depending upon the vision errors being corrected. 

* With nearsightedness -- a concave spherical lens

* With farsightedness -- a convex spherical lens

* With Astigmatism -- a cylindrical lens

* With defects of convergence -- a prism lens

Many times, a prescription calls for lenses to be ground with a combination of these forms to compensate for several vision errors together. In a glasses prescription, the higher the first number, the greater the correction in the lens. A minus (-) in front of the first number means a nearsighted prescription, while a plus (+) means a farsighted prescription.

Bifocal lenses are used to give two different amounts of vision correction in one lens. The upper portion of the lens is ground for distant vision, while the lower portion is ground for close vision and reading. This enables the wearer to simply raise their eyes to focus on distant objects, or lower them to read.

Trifocals are lenses that are ground with a special center portion to aid viewing at intermediate distances.

SUNGLASSES

Sunglasses have been popular for years as a fashion accessory and for eye comfort, but now research shows that sunglasses can protect the long-term health of your eyes. Results of many studies in the last 20 years suggest that long hours spent in the sun without adequate eye protection can increase your chances of developing certain eye problems like cataracts. Based on these studies, ophthalmologists recommend that you wear UV absorbent sunglasses and a brimmed hat whenever you're in the sun long enough to tan or burn, especially when you are near the equator or at a higher elevation. 

There are many types of sunglasses out there, and some are certainly better than others, but contrary to popular opinion, sunglasses don't have to cost a lot to do a good job of protection your eyes. Just remember to only shop for sunglasses that block 99 to 100% of UV light. Some labels say UV absorbent up to 400nm, which is the same thing as 100% UV absorption. And if the label says "special purpose" or "meets ANSI UV requirements," the lenses will block at least 99% of UV light.

If you're looking at sunglasses with other types of lenses with different benefits and features for things like sports etc. -- blue blocker, polarized, mirrored, gradient -- just make sure they also provide adequate UV protection, because some may not. And although photochromic lenses are good UV absorbent lenses that respond automatically to UV light, darkening in bright conditions and lightening in low light, the amount of time it takes them to adjust to changes in light can make them less than ideal in certain situations.

A medium density/darkness lens is perfectly adequate for day-to-day wear, but if you spend a lot of time in very bright conditions, you might want to consider a darker lens. Just remember that neither the color nor the darkness of the lens tells you anything about its most important attribute -- its ability to block UV light. It simply makes senses to wear sunglasses when it's sunny to reduce the risk of UV damage to your eyes. A gray, uniformly shaded, medium to dark density, 100% UV absorbent lens in a tight-fitting or wraparound frame is an excellent choice for everyday wear. And all of these important features can easily be found in many inexpensive pairs of sunglasses.