Prescription glasses are a medical aid used to correct refractive errors of the eye – nearsightedness, farsightedness and / or astigmatism. In addition to prescription, glasses may be protective or sunglasses.
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HISTORY How glasses were invented?
Glasses consist of a frame which holds the glass (lens), and they are worn on the nose in front of the eyes, usually to correct visual acuity, to protect eyes from UV rays and other harmful factors.
Corrective lenses adjust the focal length of the eye, in order to mitigate the effect of short-sightedness, farsightedness or astigmatism.
With aging, the lens loses its elasticity, which reduces the ability of accommodation of the eye and results in presbyopia (farsightedness). Such condition is corrected with progressive lenses which incorporate more diopters; the bottom of the lens holds the diopter for reading, and the upper part holds the diopter for distant vision.
The breaking strength of the lens is measured with diopters (D). Reading glasses (for farsightedness) have a range of lens strengths of +1.00 to +4.00 diopters.
Myopia is corrected with concave or minus lenses.
Frames are usually made of metal or plastic.
The lenses were originally made of glass, but today many are made from different types of plastic, including CR-39 and polycarbonate. These materials reduce the possibility of breakage and are lighter than glass. Some plastic materials have better optical properties than glass, such as better transmission of visible light and ultraviolet light absorption. Some also have a better refractive index than most glass lens; this is useful for the process of corrective lenses development, because they are shaped to correct various forms of visual abnormalities, myopia just being one of them, which allows the use of thinner lenses.
Newer plastic lenses, called izon, can correct some aberrations that occur naturally in the area in front of our eyes. They create sharper vision and help with haloes and flies that can be seen when the light shines on us while driving at night.
Colored and photochromic lenses are extremely useful for people who want protection from UV rays and also keep their prescription glasses on. Specifically, these lenses have the ability of getting tinted when exposed to sun light and under UV light, while indoors they remain transparent just like any other prescription glasses.
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