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Lens and glasses

Lens and glasses
Before we talk about the invention of glasses, let us recall briefly what the lens is and why it can be used to correct defects of vision.

Lens is usually called a transparent body, limited by spherical surfaces. (There are lenses, in which only one surface is spherical and the other - is flat. However, a flat surface can be regarded as spherical, if it is assumed that it has an infinitely large radius of curvature.) A well-known feature of the lens is its ability to change in a certain way the direction of the incident on light rays it. Why is this happening? Even in ancient times people have noticed that the light passing from one transparent medium to another (eg from air to water or glass), changes its direction, or, as they say, is refracted. Examples of light refraction can easily monitor each. For example, if we put a pencil in a glass of water, so that half of it is in the water and half in the air, and then look at the side of the glass, we would think that coerced by a pencil in the portion that is attributable to the air and water border. The lenses of the beam is refracted twice once entering into it, and the second time - leaving. Variously changing curvature of the lens, it is possible to achieve different refractive effects. For example, one lens can collect light at the point, while others, on the contrary, to dissipate it. Moreover, the lens in which the middle is thicker than the edges, are collected and those with thinner edges of the middle - scattering.

The point at which the light beams converge after refraction in the lens, called a focus, and the distance from the lens center to the focus - its focal length. The greater the curvature of the lens, that is less than the radius of the spherical surfaces forming a lens, the shorter its focal length. The diverging lens, too, has its focus - it is called the point at which converge the continuation of the scattered rays of the lens. The most important feature of the lens, which is based on all of its optical properties - the ability to focus light, that is to collect the rays of light emanating from a point back to one point. And since any object can be imagined as a set of infinitely many points, the lens produces not only the image of any point of the object, but the whole subject as a whole. However, in the lens image is not an exact repetition of the object - it is firstly to be reversed and, secondly, will vary in size. The reason is that the distance from the lens to the object and the distance from the lens to the image are not equal to each other. If, for example, the distance from the lens to the image five times greater than the distance from the lens to the object, the image will be five times larger than the object itself. This explains the well-known to all the lenses ability to increase the image of the object, making it easier to deal with. The greater the curvature of the lens (less than its focal length), the more it increases. If, on the contrary, the distance to the subject is greater than the distance to the image, the image is reduced.

Sharp images of objects obtained only when they are projected onto a plane passing through the focus and perpendicular to its main optical axis (the main optical axis of the lens is called a straight line passing through the centers of the spherical surfaces, limiting the lens). It is known that the human eye is an optical system. The rays of light entering the eye, the cornea and refracted on the lens surface. Lens - this transparent laminated body, like a lens. Special muscle can change the shape of the lens, making it is less, the more convex. This lens is increased, then it decreases its curvature, and with it the focus. In general, the optical system of the eye can be seen as a converging lens with a variable focal length, the image is projected onto the retina. If the subject is too far away, the image is in the normal retina, without any stress was the lens muscles. When the subject is close, there is a compression of the lens and reducing the focal length so that the image plane is aligned again with the retina. Thus, the eye is in the normal (relaxed) state, when he looks into the distance. However, in many people's eyes to create a relaxed state, the remote image of the object is not on the retina, but in front of her. As a result, the image of each object point is projected onto the retina is not at a point but as circles. Subject spreads. Such people can not see distant objects clearly, but well see those that are near. This vision defect called myopia. A similar situation arises when the image of distant objects get behind the retina. This defect is called hyperopia. Hyperopia people clearly see distant objects, but can not distinguish between those that are near. Both of these drawbacks are corrected with eyeglasses. When myopia may need glasses to spread lenses. Going through such a lens, the light rays are focused precisely on the retina by the lens. Therefore, short-sighted people, armed with glasses, can be considered distant objects, as well as people with normal vision. Hyperopia is corrected by spectacles, too, but only from the collecting lens. Points, being very simple optical device, bring people with visual defects, a huge relief in everyday life. Without glasses, these people would constantly feel their inferiority, and in highly developed myopia or hyperopia could be in the position of people with disabilities. In recent decades, when the defects of vision (especially myopia) have become extremely common, spectacles in a particular age begins to enjoy almost every person. It is therefore evident that the points shall be assigned to one of the greatest technological inventions. Although their design is very simple, there were glasses relatively late - only in medieval times, when we have learned to produce high quality, transparent and homogeneous glass. Meanwhile, with the lens properties people met much earlier - at the dawn of civilization.

In ancient times, the lenses were made of transparent stones - mainly made of rock crystal and beryl. Many of these lenses was discovered during excavations in Egypt, Greece, Mesopotamia, Italy. Several lenses found during excavations of the legendary Troy (the age of the latter is determined around 2500 BC). Clear lens made of crystal, made around 1600 BC, found in the ruins of the palace of Knossos in Crete. The very first lenses made of glass, dating from about V-IV centuries BC, found in Sargon (Mesopotamia). In more recent times, began to manufacture glass lenses more often. However, there is no mention that the lens already used as an optical instrument. Although extant several ancient writings on optics, none of them has reported that the lenses used to correct vision defects. There is not even any evidence that the lenses are used as a magnifying glass (magnifying glass) in the performance of a small work (e.g., producing gems), despite the fact that the ability of the lens to increase the image objects has, of course, well known. But why, then, were made by those old lenses, which were discussed above? Apparently, they served only as a decoration.

The first points came in the XIII century in Italy. At this time, the Italian glass master considered the world's most skilled manufacturers, polishers and glass polishers. Especially famous Venetian glass, products which were often very complex, intricate form. Constantly processing spherical, curved and convex surfaces, and then bringing them to the eyes, the wizard finally noticed optical glass features. Brilliant idea to combine two lenses with the help of the frame came, according to legend, in 1285 the glass master Salvino Armati from Florence. He also established the first production points. Nothing more about this man is not known. However, he invented a device that allows easy and quick to smooth defects of vision, but now has spread. In the very first long-focus glasses inserted convex, collecting lens, and they were used to correct farsightedness. Much later it was discovered that by using the same points, inserting them concave diverging lenses can correct nearsightedness. The first descriptions of these points apply only to the XVI century.

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