Optical glass is a specially made transparent glass that is used as parts for optical instruments. It differs from the usual purity and increased transparency, uniformity and colorlessness. Also, dispersion and refractive power are strictly normalized in it. Compliance with such requirements increases the complexity and cost of production.
History
You can find many examples of everyday use of lenses, for example, a magnifier - an ordinary magnifying glass - will help to create a small projector from an ordinary smartphone, but optical glasses appeared not so long ago.
Lenses are known from antiquity, but the first serious attempt to create glass, similar to that used in modern devices, can be attributed to the XVII century. So, the German chemist Kunkel in one of his works mentioned phosphoric and boric acids that are part of the glass component. He also spoke of a borosilicate crown, which is close to some modern materials in composition. This can be called the first successful experience in the production of glass with certain optical properties and a sufficient degree of physical and chemical homogeneity.
In industry
The manufacture of optical glasses on an industrial scale began at the beginning of the 19th century. Swiss Gian, together with Fraunhofer, introduced a relatively stable method for producing such glass at a factory in Bavaria. The key to success was the method of mixing the melt using circular motions of a clay rod vertically immersed in glass. As a result, it was possible to obtain optical glass of satisfactory quality, with a diameter of up to 250 mm.
Modern production
In obtaining non-ferrous optical glasses, additives of substances containing copper, selenium, gold, silver and other metals are used. Cooking comes from the mixture. It is loaded into refractory pots, which, in turn, are placed in a glass melting furnace. The composition of the charge may include up to 40% of glass waste, an important point is the composition of cullet and boiling glass. The glass melt is continuously mixed during cooking with a ceramic spatula or platinum. Thus a homogeneous state is achieved.
Periodically, the melt is taken for a sample, according to which quality is controlled. An important stage in cooking is clarification: in a glass mass of substances-clarifiers, originally added to the composition of the mixture, the release of a significant amount of gas begins. Large bubbles form, which rise quickly, capturing smaller bubbles, which inevitably form during the cooking process.
At the end, the pots are removed from the furnace, and then slowly cool. Cooling, slowed down by special techniques, can last up to eight days. It must be uniform, otherwise mechanical stress can form in the mass, which causes cracks.
The properties
Optical glass is a lens material. They, in turn, are divided by type of collecting and scattering. Collecting lenses include a biconvex and plano-convex lens, as well as a concave-convex lens, called the “positive meniscus”.
Optical glass has a number of characteristics:
- refractive index, determined by two spectral lines, which are called sodium doublet;
- average dispersion, which is understood as the difference between the refraction of the red and blue lines of the spectrum;
- dispersion coefficient - the number given by the ratio of the average dispersion and refraction.
Colored optical glass is used to produce absorption filters. Three main types of optical glasses are distinguished depending on the material:
- inorganic;
- plexiglass (organic);
- mineral and organic.
Inorganic glass contains oxides and fluorides. Quartz optical glass also refers to inorganic (chemical formula SiO 2 ). Quartz has a small refraction and a high transmittance, it is characterized by heat resistance. A wide range of transparency allows it to be used in modern telecommunications (fiber optic cables, etc.), and silicate glass is indispensable in the manufacture of optical lenses, for example, magnifying glass is made of quartz.
Silicon based
Transparent silicate glass can be both optical and technical. Optical is made by melting rock crystal, only this way a completely homogeneous structure is obtained. In opaque glasses, small gas bubbles inside the material are responsible for color.
In addition to silica glass, silicon-based is also used to produce so-called silicon glass, which, despite having a similar base, has other optical properties. Silicon elements are able to refract X-rays and transmit infrared radiation.
Organic glass
The so-called plexiglass is made on the basis of synthetic polymeric material. This transparent and hard material belongs to thermoplastics and is often used as a replacement for quartz glass. Plexiglass is resistant to many environmental factors, such as high humidity and low temperatures, but it is much softer, and therefore more sensitive to mechanical stress. Due to its softness, organic optical glass is easy to process - even the simplest metal cutting tool can take it.
This material is great for laser processing, it is easy to apply a pattern or engraving. As a lens, it perfectly reflects infrared rays, but transmits ultraviolet and X-rays.
Application
Optical glasses are widely used for the manufacture of lenses, which, in turn, are used in many optical systems. A single collecting lens is used as a magnifying glass. In technology, lenses are an important or main part of such systems as binoculars, optical sights, microscopes, theodolites, telescopes, as well as cameras and video equipment.
No less important are optical glasses for the needs of ophthalmology, because without them it is difficult or impossible to correct visual impairments (myopia, astigmatism, hyperopia, disturbance of accommodation and other diseases). Lenses for glasses with diopters can be made both from quartz glass and from high-quality plastic.
Astronomy
Optical glasses are an important and most expensive component of any telescope. Many lovers themselves collect refractors, this requires a little, but most importantly - a flat-convex glass lens.
At the beginning of the century before last, it took several years to produce one powerful astronomical lens, or rather to polish it. For example, in 1982, the head of the University of Chicago William Harper asked the millionaire Charles Yerks to fund the observatory. Yerks invested in it about three hundred thousand dollars, and forty thousand went to buy a lens for the most powerful telescope at that time on the planet. The observatory was named after the financier Yerks, and still this refractor with a lens diameter of 102 cm is considered the largest in the world.
Large-diameter telescopes are reflectors, in it the mirror is a light-collecting element.
There is another type of lens that is used both in astronomy and in ophthalmology - glass with convex-concave surfaces, which is called the meniscus. It can be of two types: scattering and collecting. In the scattering meniscus, the extreme part is thicker than the central part, and in the collecting meniscus the central part is thinner.