Original Article: https://en.wikipedia.org/wiki/Glasses
Glasses, also known as eyeglasses or spectacles, are devices consisting of lenses mounted in a frame that holds them in front of a person's eyes. Glasses are typically used for vision correction. Safety glasses provide eye protection against flying debris or against visible and near-visible light or radiation. Sunglasses allow better vision in bright daylight, and may protect one's eyes against damage from high levels of ultraviolet light. Specialized glasses may be used for viewing specific visual information (such as stereoscopy). Sometimes glasses are worn simply for aesthetic or fashion purposes. The number of Americans who are nearsighted has doubled since the 1970s and almost 3/4 of the US population wears glasses. People are more likely to need glasses the older they get with 93% of people between the age of 65-75 wearing corrective lenses.
Corrective lenses are used to correct refractive errors by bending the light entering the eye in order to alleviate the effects of conditions such as nearsightedness (myopia), farsightedness (Hypermetropia) or astigmatism. Another common condition in patients over forty years old is presbyopia, which is caused by the eye's crystalline lens losing elasticity, progressively reducing the ability of the lens to accommodate (i.e. to focus on objects close to the eye). Corrective lenses are made to conform to the prescription of an ophthalmologist or optometrist. A lensmeter can be used to verify the specifications of a pair of glasses.
Pinhole glasses are a type of corrective glasses that do not use a lens. Pinhole glasses do not actually refract the light or change focal length. Instead, they create a diffraction limited system, which has an increased depth of field, similar to using a small aperture in photography. This form of correction has many limitations that prevent it from gaining popularity in everyday use.
Corrective eyeglasses can significantly improve the life quality of the wearer. Not only do they enhance the wearer's visual experience, but can also reduce problems that appear such as headaches or squinting.
The most common type of corrective lens is "single vision", which has a uniform refractive index. For people with presbyopia and hyperopia, bifocal and trifocal glasses provide two or three different refractive indexes, respectively, and progressive lenses have a continuous gradient. Reading glasses provide a separate set of glasses for focusing on close-by objects. Adjustable-focus eyeglasses might be used to replace bifocals or trifocals, or might be used to produce cheaper single-vision glasses (since they don't have to be custom-manufactured for every person).