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Eye Glasses Lenses: Bi Focal, Tri Focal & More
Eyeglasses or spectacles, though most commonly, glasses, are frames which bear lenses worn before the eyes. They are normally used for visual correction, eye protection or otherwise protection from ultra violent rays from the sun. Modern glasses designs are normally supported by two pads cornering the bridge area of the nose as well as two arms that are held up by the backs of the ears to hold up, center and balance the glasses.
Glasses frames are made of either metal or plastic; the eye lenses are sometimes made from treated glass, but are being more and more replaced with different kinds of hard plastic such as CR-19 and less commonly, polycarbonate. These materials replacing glass help reduce and sometimes eliminate the threat of the lenses breaking and also weight much less than actual glass lenses. IN some other plastics, more optical advantage properties are present to them that make them superior to many glass lenses. 
Some of these advantageous include greater absorption of UV light as well as the better transmission of most visible lights, and a somewhat greater area of refraction than many kinds of glass. These types of plastic are able to correct many types of abnormalities of vision. This, in turn, allows for the prescription of thinner lenses and, thus, lighter more manageable glasses. Some newer plastic lenses known as Izon can also aid and correct for the higher aberrations that can naturally occur over the surfaces of the eye. These types of lenses can create sharper vision and also can help with some night time vision problems such as halos, comet tails, starbursts that can occur in the eyes.
What are the different Powers and Tints of Glasses?
Eye glasses power and tint depends on the various types of glasses that are being used. There are many instances in which a set of eye glasses need to be used, and for many occasions, there are different types of glasses that are used for different purposes. Most glasses are subcategorized into several distinct types. The different types include;
Corrective - These are used to correct some types of eye errors by modifying glasses lens to alleviate nearsightedness, farsightedness or astigmatism. These are made from different tints of glass and are sometimes tinted slightly for some conditions. These are moderate level optical lenses.
Safety - These glasses do not have any particular benefit to the optical functioning of the eye. They are made from clear plastic and used only for the protection of the eye from hazardous materials. 
Sun/ UV protection- These glasses are usually worn for fashionable or recreational means, however, there are some optical conditions that require safe guard against UV rays. These are made from a graphite plastic that is tinted to darken and withdraw light entering into the eye. 
Reading - These glasses are us