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. It also strictly normalizes the dispersion and refractive power. 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 magnifying glass is an ordinary magnifying glass - will help you create a small projector from an ordinary smartphone, but optical glasses have appeared not so long ago.
Lenses have been known since antiquity, but the first serious attempt to create glass similar to that used in modern devices can be traced back to the 17th century. So, the German chemist Kunkel in one of his works mentioned phosphoric and boric acids, which are part of the glass component. He also talked about borosilicate crown, which is close to some modern materials in terms of composition. This can be called the first successful experience in the production of glass with certain optical properties and a sufficient degree ofphysical and chemical homogeneity.
In industry
The production of optical glasses on an industrial scale began at the beginning of the 19th century. The Swiss Gian, together with Fraunhofer, introduced a relatively stable method for producing such glass at one of the plants in Bavaria. The key to success was the technique of mixing the melt with the help of circular movements of a clay rod vertically immersed in glass. As a result, it was possible to obtain optical glass of satisfactory quality, up to 250 mm in diameter.
Modern production
In the production of colored 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 furnace. The composition of the charge can include up to 40% of glass waste, an important point is the compliance of the composition of cullet and melting glass. Glass mass during cooking is continuously mixed with a spatula made of ceramic or platinum. In this way a uniform state is achieved.
Periodically, the melt is taken for a sample, which controls the quality. An important stage of melting is clarification: in the glass mass, the release of a significant amount of gases begins from the clarifying substances that were initially added to the mixture. Large bubbles form and rise quickly, trapping the smaller bubbles that inevitably form during the brewing process.
Finally, the pots are removed from the oven, after whichcool down slowly. Cooling, slowed down by special techniques, can last up to eight days. It must be uniform, otherwise mechanical stresses may form in the mass, which cause cracks.
Properties
Optical glass is a material for the production of lenses. They, in turn, are divided by type into collecting and scattering. Collecting lenses include biconvex and plano-convex lenses, as well as concave-convex, called the "positive meniscus".
Optical glass has a number of characteristics:
- refractive index determined by two spectral lines called the sodium doublet;
- average dispersion, which is understood as the difference between the refraction of the red and blue lines of the spectrum;
- dispersion coefficient - a number given by the ratio of average dispersion and refraction.
Colored optical glass is used for the production of absorption filters. Depending on the material, there are three main types of optical glasses:
- inorganic;
- plexiglass (organic);
- mineral-organic.
Inorganic glass contains oxides and fluorides. Quartz optical glass also belongs to inorganic (chemical formula SiO2). Quartz has low refraction and high light transmission, it is characterized by heat resistance. A wide range of transparency allows it to be used in moderntelecommunications (optical fiber cables, etc.), also silicate glass is indispensable in the manufacture of optical lenses, for example, a magnifying glass is made from quartz.
Based on silicon
Transparent silicate glass can be both optical and technical. Optical is made by melting rock crystal, only in this way a completely homogeneous structure is obtained. In opaque glasses, small gas bubbles inside the material are responsible for the color.
In addition to silicon-based quartz glass, the so-called silicon glass is also produced, which, despite a similar base, has different optical properties. Silicon cells are able to refract X-rays and transmit infrared radiation.
Organic glass
The so-called plexiglass is made on the basis of a synthetic polymer material. This transparent and hard material belongs to thermoplastics and is often used as a replacement for quartz glass. Plexiglas 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 and easy to pattern or engrave. As a lens, it perfectly reflects infrared rays, buttransmits 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 converging 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.
Optical glasses are no less important for the needs of ophthalmology, because without them it is difficult or impossible to correct visual impairments (nearsightedness, astigmatism, farsightedness, disturbance of accommodation and other diseases). Spectacle lenses with diopters can be made from both quartz glass and high-quality plastic.
Astronomy
Optical glasses are the most important and most expensive component of any telescope. Many hobbyists assemble their own refractors, it requires little, but the most important thing is a plano-convex glass lens.
At the beginning of the century before last, it took several years to make one powerful astronomical lens, or rather to polish it. For example, in 1982, the head of the University of Chicago, William Harper, approached the millionaire Charles Yerkes with a request to finance the observatory. Yerkes invested about three hundred thousand dollars in it, with forty thousand spent on buying a lens for the most powerful telescope on the planet at that time. The observatory was named after the financier Yerkes, and to this day this refractor with a lens diameter of 102cm is considered the largest in the world.
Telescopes with a large diameter are reflectors, in which the mirror is a light-collecting element.
There is another type of lens used in both astronomy and 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 one, and in the collecting meniscus, the central part is thinner.
