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Eyeglasses Eyeglasses
Also called just ‘glasses’, or spectacles, eyeglasses are lenses mounted in a frame designed to rest in front of the eyes and
correct problems with vision. The use of eyeglasses began in China and Europe in ancient times. At first, natural crystals of quartz were used, then curved glass. Eyeglasses improved with the knowledge of how lenses worked and with better technology for casting curved glass and polishing it to the required
shape. Places like Venice, in Italy, and Nuremberg, in Germany, that were originally famous for their glassware later became known for their eyeglasses.
The first glasses used bulging (convex) lenses and corrected for long-sightedness. It was only in the 16th century that dished (concave) lenses were developed for shortsightedness (myopia).
In 1784 Benjamin Franklin invented bifocals, making the upper part of the lens suitable for use
when looking in the distance and the lower part useful for reading.
Modern lenses begin as a sheet of plate glass and are then shaped by using fine abrasive powder and specially shaped tools. Many lenses are now made of plastic. Since plastic is less dense than glass, plastic eyeglasses are not as heavy. Plastic scratches more easily than glass, but it is less brittle than glass and less likely to shatter.
Contact lenses are small lenses held on the surface of the eye. They are made from either glass or plastic.
Sunglasses often contain no corrective lenses at all and are simply coated with material that reduces the light passing through the lens. Tinted coating can also be applied to prescription glasses.
F
Fibre optics
A way of sending light over
long distances through transparent fibres. The fibres can be glass or plastic. It does not matter whether the fibres are straight or curved.
Focuses in front of retina
Far object
Shortsighted people cannot see things clearly in focus if they are far away.
Concave lens in eyeglasses
 Eyeglasses – A concave lens is used for people who cannot see things in focus when they are at a distance. In this case the lens of the eye brings light to a focus in front of the back of the eye. The glass lens slightly diverges the light, so that the eye lens can then bring objects to a focus on the back of the eye.
 Eyeglasses – A convex lens is used for people who cannot see things in focus when they are very close. This is the more usual eye problem. In this case the lens of the eye brings light to a focus behind the back of the eye. The glass lens slightly converges the light, so that the eye lens can then bring objects to a focus on the back of the eye.
Near object (less than 25cm)
Farsighted people cannot see things clearly in focus if they are close up.
Focuses behind retina
Convex lens in eyeglasses
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