Rear-projection television is a type of large-screen television display technology. Until 2006, most of the affordable consumer large screen TVs up to 100 in used rear-projection technology. A variation is a video projector. Three types of projection systems are used in projection TVs. CRT rear-projection TVs were the earliest, while they were the first to exceed 40", they were bulky and the picture was unclear at close range. Newer technologies include: DLP, LCD projectors, Laser TV and LCoS, they are capable of 1080p resolution, examples include Sony's SXRD, JVC's D-ILA, MicroDisplay Corporation's Liquid Fidelity. Projection systems were popular from 1946 through 1948 when it was still difficult to manufacture CRTs with a screen size much over 12 inches. Using a 3, 4 or 5 inch monochrome CRT driven at a high accelerating voltage for the size, the tube produced an bright picture, projected via a Schmidt lens and mirror assembly onto a semi translucent screen of 22.5 to 30 inches diagonal in size.
The resultant picture was darker than with a direct view CRT and had to be watched in subdued lighting. The degree to which the tube was driven meant that the tube had a short life. Details of a specific TV set with its optical system can be found here. Modern rear-projection television has been commercially available since the 1970s, but at that time could not match the image sharpness of a direct-view CRT. Current models are vastly improved, offer a cost-effective HDTV large-screen display. While still thicker than LCD and plasma flat panels, modern rear-projection TVs have a smaller footprint than their predecessors; the latest models are thin and light enough to be wall-mounted, although by this time the market for rear-projection TVs was declining. Given their large dimensions, projection TVs sometimes included larger speakers and more powerful built-in audio vs direct view CRTs and depth-limited flat panels, as well as basic surround sound processing or emulators such as Sound Retrieval System by SRS Labs, similar to a sound bar.
While popular in the early 2000s as an alternative to more expensive LCD and plasma flat panels, the falling price and improvements to LCDs led to Sony, Philips and Hitachi dropping rear-projection TVs from their lineup. Samsung, ProScan, RCA, Panasonic and JVC exited the market as LCD televisions became the standard; the bulk of earlier rear-projection TVs meant that they cannot be wall-mounted, while most consumers of flat-panels do not hang up their sets, the ability to do so is considered a key selling point. On June 6, 2007, Sony did unveil a 70" rear-projection SXRD model KDS-Z70XBR5, 40% slimmer than its predecessor and weighed 200 lbs, somewhat wall-mountable. However, on December 27, 2007, Sony decided to exit the RPTV market. Mitsubishi began offering their LaserVue line of wall mountable rear-projection TVs in 2009. A projection television uses a projector to create a small image or video from a video signal and magnify this image onto a viewable screen; the projector uses a bright beam of light and a lens system to project the image to a much larger size.
A front-projection television uses a projector, separate from the screen and the projector is placed in front of the screen. The setup of a rear-projection television is in some ways similar to that of a traditional television; the projector projects the image from behind the screen. The following are different types of projection televisions, which differ based on the type of projector and how the image is created: CRT projector: Small cathode ray tubes create the image in the same manner that a traditional CRT television does, by firing a beam of electrons onto a phosphor-coated screen and the image is projected to a large screen; this is done to overcome the limit of size of cathode ray tube, about 40 inches. 3 CRTs are used, one red, one green and one blue, aligned so the colors mix on the projected image. LCD projector: A lamp transmits light through a small LCD chip made up of individual pixels to create an image; the LCD projector uses mirrors to take the light and create three separate red and blue beams, which are passed through three separate LCD panels.
The liquid crystals are manipulated using electric current to control the amount of light passing through. The lens system takes the three color projects the image. Digital Light Processing projector: A DLP projector creates an image using a digital micromirror device, which on its surface contains a large matrix of microscopic mirrors, each corresponding to one pixel in an image; each mirror can be rotated to reflect light such that the pixel appears bright, or the mirror can be rotated to direct light elsewhere and make the pixel appear dark. The mirror is rotated on an axle hinge. There are electrodes on both sides of the hinge controlling the rotation of the mirror using electrostatic attraction; the electrodes are connected to an SRAM cell located under each pixel, charges from the SRAM cell drive the movement of the mirrors. Color is added to the image-creation process either through a spinning color wheel or a three-chip projector; the color wheel is placed between the lamp light source and the DMD chip such that the light passing through is colored and reflected off a mirror to determine the level of darkness.
A color wheel consists
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths; when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.
A mirror is an object that reflects light in such a way that, for incident light in some range of wavelengths, the reflected light preserves many or most of the detailed physical characteristics of the original light, called specular reflection. This is different from other light-reflecting objects that do not preserve much of the original wave signal other than color and diffuse reflected light, such as flat-white paint; the most familiar type of mirror is the plane mirror. Curved mirrors are used, to produce magnified or diminished images or focus light or distort the reflected image. Mirrors are used for personal grooming or admiring oneself, for viewing the area behind and on the sides on motor vehicles while driving, for decoration, architecture. Mirrors are used in scientific apparatus such as telescopes and lasers and industrial machinery. Most mirrors are designed for visible light. There are many types of glass mirrors, each representing a different manufacturing process and reflection type.
An aluminium glass mirror is made of a float glass manufactured using vacuum coating, i.e. aluminium powder is evaporated onto the exposed surface of the glass in a vacuum chamber and coated with two or more layers of waterproof protective paint. A low aluminium glass mirror is manufactured by coating silver and two layers of protective paint on the back surface of glass. A low aluminium glass mirror is clear, light transmissive and reflects accurate natural colors; this type of glass is used for framing presentations and exhibitions in which a precise color representation of the artwork is essential or when the background color of the frame is predominantly white. A safety glass mirror is made by adhering a special protective film to the back surface of a silver glass mirror, which prevents injuries in case the mirror is broken; this kind of mirror is used for furniture, glass walls, commercial shelves, or public areas. A silkscreen printed glass mirror is produced using inorganic color ink that prints patterns through a special screen onto glass.
Various colors and glass shapes are available. Such a glass mirror is durable and more moisture resistant than ordinary printed glass and can serve for over 20 years; this type of glass is used for decorative purposes. A silver glass mirror is an ordinary mirror, coated on its back surface with silver, which produces images by reflection; this kind of glass mirror is produced by coating a silver, copper film and two or more layers of waterproof paint on the back surface of float glass, which resists acid and moisture. A silver glass mirror provides clear and actual images, is quite durable, is used for furniture and other decorative purposes. Decorative glass mirrors are handcrafted. A variety of shades and glass thickness are available. A beam of light reflects off a mirror at an angle of reflection equal to its angle of incidence; that is, if the beam of light is shining on a mirror's surface, at a θ ° angle vertically it reflects from the point of incidence at a θ ° angle, vertically in the opposite direction.
This law mathematically follows from the interference of a plane wave on a flat boundary. In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel. In a concave mirror, parallel beams of light become a convergent beam, whose rays intersect in the focus of the mirror. Known as converging mirror In a convex mirror, parallel beams become divergent, with the rays appearing to diverge from a common point of intersection "behind" the mirror. Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. However, the ideal of focusing to a point is a used approximation. Parabolic reflectors resolve this. Parabolic reflectors are not suitable for imaging nearby objects because the light rays are not parallel. Objects viewed in a mirror will appear not vertically inverted. However, a mirror does not "swap" left and right any more than it swaps top and bottom. A mirror reverses the forward/backward axis. To be precise, it reverses the object in the direction perpendicular to the mirror surface.
Because left and right are defined relative to front-back and top-bottom, the "flipping" of front and back results in the perception of a left-right reversal in the image. Looking at an image of oneself with the front-back axis flipped results in the perception of an image with its left-right axis flipped; when reflected in the mirror, your right hand remains directly opposite your real right hand, but it is perceived as the left hand of your image. When a person looks into a mirror, the image is front-back reversed, an effect similar to the holl
Glass is a non-crystalline, amorphous solid, transparent and has widespread practical and decorative uses in, for example, window panes and optoelectronics. The most familiar, the oldest, types of manufactured glass are "silicate glasses" based on the chemical compound silica, the primary constituent of sand; the term glass, in popular usage, is used to refer only to this type of material, familiar from use as window glass and in glass bottles. Of the many silica-based glasses that exist, ordinary glazing and container glass is formed from a specific type called soda-lime glass, composed of 75% silicon dioxide, sodium oxide from sodium carbonate, calcium oxide called lime, several minor additives. Many applications of silicate glasses derive from their optical transparency, giving rise to their primary use as window panes. Glass will transmit and refract light. Glass can be coloured by adding metallic salts, can be painted and printed with vitreous enamels; these qualities have led to the extensive use of glass in the manufacture of art objects and in particular, stained glass windows.
Although brittle, silicate glass is durable, many examples of glass fragments exist from early glass-making cultures. Because glass can be formed or moulded into any shape, it has been traditionally used for vessels: bowls, bottles and drinking glasses. In its most solid forms it has been used for paperweights and beads; when extruded as glass fiber and matted as glass wool in a way to trap air, it becomes a thermal insulating material, when these glass fibers are embedded into an organic polymer plastic, they are a key structural reinforcement part of the composite material fiberglass. Some objects were so made of silicate glass that they are called by the name of the material, such as drinking glasses and eyeglasses. Scientifically, the term "glass" is defined in a broader sense, encompassing every solid that possesses a non-crystalline structure at the atomic scale and that exhibits a glass transition when heated towards the liquid state. Porcelains and many polymer thermoplastics familiar from everyday use are glasses.
These sorts of glasses can be made of quite different kinds of materials than silica: metallic alloys, ionic melts, aqueous solutions, molecular liquids, polymers. For many applications, like glass bottles or eyewear, polymer glasses are a lighter alternative than traditional glass. Silicon dioxide is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, branching rootlike structures called fulgurites. Fused quartz is a glass made from chemically-pure silica, it has excellent resistance to thermal shock, being able to survive immersion in water while red hot. However, its high melting temperature and viscosity make it difficult to work with. Other substances are added to simplify processing. One is sodium carbonate; the soda makes the glass water-soluble, undesirable, so lime, some magnesium oxide and aluminium oxide are added to provide for a better chemical durability. The resulting glass is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.
Most common glass contains other ingredients to change its properties. Lead glass or flint glass is more "brilliant" because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eyeglasses. Iron can be incorporated into glass to absorb infrared radiation, for example in heat-absorbing filters for movie projectors, while cerium oxide can be used for glass that absorbs ultraviolet wavelengths; the following is a list of the more common types of silicate glasses and their ingredients and applications: Fused quartz called fused-silica glass, vitreous-silica glass: silica in vitreous, or glass, form. It has low thermal expansion, is hard, resists high temperatures, it is the most resistant against weathering. Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.
Soda-lime-silica glass, window glass: silica + sodium oxide + lime + magnesia + alumina. Is transparent formed and most suitable for window glass, it has a high thermal expansion and poor resistance to heat. It is used for windows, some low-temperature incandescent light bulbs, tableware. Container glass is a soda-lime glass, a slight variation on flat glass, which uses more alumina and calcium, less sodium and magnesium, which are more water-soluble; this makes it less susceptible to water erosion. Sodium borosilicate glass, Pyrex: silica + boron trioxide + soda + alumina. Stan
Beryllium is a chemical element with symbol Be and atomic number 4. It is a rare element in the universe occurring as a product of the spallation of larger atomic nuclei that have collided with cosmic rays. Within the cores of stars beryllium is depleted as it creates larger elements, it is a divalent element which occurs only in combination with other elements in minerals. Notable gemstones which contain beryllium include chrysoberyl; as a free element it is a steel-gray, strong and brittle alkaline earth metal. Beryllium improves many physical properties when added as an alloying element to aluminium, copper and nickel. Beryllium does not form oxides until it reaches high temperatures. Tools made of beryllium copper alloys are strong and hard and do not create sparks when they strike a steel surface. In structural applications, the combination of high flexural rigidity, thermal stability, thermal conductivity and low density make beryllium metal a desirable aerospace material for aircraft components, missiles and satellites.
Because of its low density and atomic mass, beryllium is transparent to X-rays and other forms of ionizing radiation. The high thermal conductivities of beryllium and beryllium oxide have led to their use in thermal management applications; the commercial use of beryllium requires the use of appropriate dust control equipment and industrial controls at all times because of the toxicity of inhaled beryllium-containing dusts that can cause a chronic life-threatening allergic disease in some people called berylliosis. Beryllium is a steel gray and hard metal, brittle at room temperature and has a close-packed hexagonal crystal structure, it has a reasonably high melting point. The modulus of elasticity of beryllium is 50% greater than that of steel; the combination of this modulus and a low density results in an unusually fast sound conduction speed in beryllium – about 12.9 km/s at ambient conditions. Other significant properties are high specific heat and thermal conductivity, which make beryllium the metal with the best heat dissipation characteristics per unit weight.
In combination with the low coefficient of linear thermal expansion, these characteristics result in a unique stability under conditions of thermal loading. Occurring beryllium, save for slight contamination by the cosmogenic radioisotopes, is isotopically pure beryllium-9, which has a nuclear spin of 3/2. Beryllium has a large scattering cross section for high-energy neutrons, about 6 barns for energies above 10 keV. Therefore, it works as a neutron reflector and neutron moderator slowing the neutrons to the thermal energy range of below 0.03 eV, where the total cross section is at least an order of magnitude lower – exact value depends on the purity and size of the crystallites in the material. The single primordial beryllium isotope 9Be undergoes a neutron reaction with neutron energies over about 1.9 MeV, to produce 8Be, which immediately breaks into two alpha particles. Thus, for high-energy neutrons, beryllium is a neutron multiplier, releasing more neutrons than it absorbs; this nuclear reaction is: 94Be + n → 2 42He + 2 nNeutrons are liberated when beryllium nuclei are struck by energetic alpha particles producing the nuclear reaction 94Be + 42He → 126C + n, where 42He is an alpha particle and 126C is a carbon-12 nucleus.
Beryllium releases neutrons under bombardment by gamma rays. Thus, natural beryllium bombarded either by alphas or gammas from a suitable radioisotope is a key component of most radioisotope-powered nuclear reaction neutron sources for the laboratory production of free neutrons. Small amounts of tritium are liberated when 94Be nuclei absorb low energy neutrons in the three-step nuclear reaction 94Be + n → 42He + 62He, 62He → 63Li + β−, 63Li + n → 42He + 31HNote that 62He has a half-life of only 0.8 seconds, β− is an electron, 63Li has a high neutron absorption cross-section. Tritium is a radioisotope of concern in nuclear reactor waste streams; as a metal, beryllium is transparent to most wavelengths of X-rays and gamma rays, making it useful for the output windows of X-ray tubes and other such apparatus. Both stable and unstable isotopes of beryllium are created in stars, but the radioisotopes do not last long, it is believed that most of the stable beryllium in the universe was created in the interstellar medium when cosmic rays induced fission in heavier elements found in interstellar gas and dust.
Primordial beryllium contains only one stable isotope, 9Be, therefore beryllium is a monoisotopic element. Radioactive cosmogenic 10Be is produced in the atmosphere of the Earth by the cosmic ray spallation of oxygen. 10Be accumulates at the soil surface, where its long half-life permits a long residence time before decaying to boron-10. Thus, 10Be and its daughter products are used to examine natural soil erosion, soil formation and the development of lateritic soils, as a proxy for measurement of the variations in solar activity and the age of ice cores; the production of 10Be is inversely proportional to solar activity, because increased solar wind during periods of high solar activity decreases the flux of galactic cosmic rays that reach the Earth. Nuclear explosions form 10Be by the reaction of fast neutrons with 13C in the carbon dioxide in air; this is one of the indicators of past activity at nuclear weapon
Gold is a chemical element with symbol Au and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright reddish yellow, soft and ductile metal. Chemically, gold is a group 11 element, it is solid under standard conditions. Gold occurs in free elemental form, as nuggets or grains, in rocks, in veins, in alluvial deposits, it occurs in a solid solution series with the native element silver and naturally alloyed with copper and palladium. Less it occurs in minerals as gold compounds with tellurium. Gold is resistant to most acids, though it does dissolve in aqua regia, a mixture of nitric acid and hydrochloric acid, which forms a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to refine gold and to confirm the presence of gold in metallic objects, giving rise to the term acid test. Gold dissolves in alkaline solutions of cyanide, which are used in mining and electroplating.
Gold dissolves in mercury, forming amalgam alloys. A rare element, gold is a precious metal, used for coinage and other arts throughout recorded history. In the past, a gold standard was implemented as a monetary policy, but gold coins ceased to be minted as a circulating currency in the 1930s, the world gold standard was abandoned for a fiat currency system after 1971. A total of 186,700 tonnes of gold exists above ground, as of 2015; the world consumption of new gold produced is about 50% in jewelry, 40% in investments, 10% in industry. Gold's high malleability, resistance to corrosion and most other chemical reactions, conductivity of electricity have led to its continued use in corrosion resistant electrical connectors in all types of computerized devices. Gold is used in infrared shielding, colored-glass production, gold leafing, tooth restoration. Certain gold salts are still used as anti-inflammatories in medicine; as of 2017, the world's largest gold producer by far was China with 440 tonnes per year.
Gold is the most malleable of all metals. It can be drawn into a monoatomic wire, stretched about twice before it breaks; such nanowires distort via formation and migration of dislocations and crystal twins without noticeable hardening. A single gram of gold can be beaten into a sheet of 1 square meter, an avoirdupois ounce into 300 square feet. Gold leaf can be beaten thin enough to become semi-transparent; the transmitted light appears greenish blue, because gold reflects yellow and red. Such semi-transparent sheets strongly reflect infrared light, making them useful as infrared shields in visors of heat-resistant suits, in sun-visors for spacesuits. Gold is a good conductor of electricity. Gold has a density of 19.3 g/cm3 identical to that of tungsten at 19.25 g/cm3. By comparison, the density of lead is 11.34 g/cm3, that of the densest element, osmium, is 22.588±0.015 g/cm3. Whereas most metals are gray or silvery white, gold is reddish-yellow; this color is determined by the frequency of plasma oscillations among the metal's valence electrons, in the ultraviolet range for most metals but in the visible range for gold due to relativistic effects affecting the orbitals around gold atoms.
Similar effects impart a golden hue to metallic caesium. Common colored gold alloys include the distinctive eighteen-karat rose gold created by the addition of copper. Alloys containing palladium or nickel are important in commercial jewelry as these produce white gold alloys. Fourteen-karat gold-copper alloy is nearly identical in color to certain bronze alloys, both may be used to produce police and other badges. White gold alloys can be made with nickel. Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. Blue gold can be made by alloying with iron, purple gold can be made by alloying with aluminium. Less addition of manganese, aluminium and other elements can produce more unusual colors of gold for various applications. Colloidal gold, used by electron-microscopists, is red. Gold has only one stable isotope, 197Au, its only occurring isotope, so gold is both a mononuclidic and monoisotopic element. Thirty-six radioisotopes have been synthesized, ranging in atomic mass from 169 to 205.
The most stable of these is 195Au with a half-life of 186.1 days. The least stable is 171Au. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, β+ decay; the exceptions are 195Au, which decays by electron capture, 196Au, which decays most by electron capture with a minor β− decay path. All of gold's radioisotopes with atomic masses above 197 decay by β− decay. At least 32 nuclear isomers have been characterized, ranging in atomic mass from 170 to 200. Within that range, only 178Au, 180Au, 181Au, 182Au, 188Au do not have isomers. Gold's most stable isomer is 198m2Au with a half-life of 2.27 days. Gold's least stable isomer is 177m2Au with a half-life of only 7 ns. 184m1Au has three decay paths: β+ decay, isomeric
Silvering is the chemical process of coating glass with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were silvered with an amalgam of tin and mercury, but by the 19th century, mirrors were made through a process by which silver was coated onto a glass surface. Today, sputtering aluminium or other compounds is more used for this purpose, although the process may maintain the name "silvering"; the earliest mirrors were made from polished obsidian during the Stone Age. By the Bronze Age most cultures had adopted mirrors made from polished discs of bronze, copper or other metals; such metal mirrors remained the norm through to Greco-Roman Antiquity and throughout the Middle Ages in Europe. In the 1st century CE glass mirrors began to appear, now believed to have originated in Sidon in present-day Lebanon. Ptolemaic Egypt had manufactured small glass mirrors backed by tin, or antimony. In the early 10th century, the Persian scientist al-Razi described ways of silvering and gilding in a book on alchemy, but this was not done for the purpose of making mirrors.
From the 15th century to 1900, tin-mercury amalgam was used in European mirror manufacture. The thin tinfoil used to silver mirrors was known as "tain". In 1835 German chemist Justus von Liebig developed a process for depositing silver on the rear surface of a piece of glass; the process was made easier by French chemist Tony Petitjean. This reaction is a variation of the Tollens' reagent for aldehydes. A diamminesilver solution is sprayed onto the glass surface; the sugar is oxidized by silver, itself reduced to silver, i.e. elemental silver, deposited onto the glass. In 1856-57 Karl August von Steinheil and Léon Foucault introduced the process of depositing an ultra-thin layer of silver on the front surface of a piece of glass, making the first optical-quality first surface glass mirrors, replacing the use of speculum metal mirrors in reflecting telescopes; these techniques soon became standard for technical equipment. An aluminum vacuum-deposition process invented in 1930 by Caltech physicist and astronomer John Strong, led to most reflecting telescopes shifting to aluminum.
Some modern telescopes use silver, such as the Kepler space observatory. The Kepler mirror's silver was deposited using ion assisted evaporation. In modern aluminum silvering, a sheet of glass is placed in a vacuum chamber with electrically heated nichrome coils that can evaporate aluminum. In a vacuum, the hot aluminum atoms travel in straight lines; when they hit the surface of the mirror, they stick. Some mirror makers evaporate a layer of beryllia on the mirror. Mirrors made by this method are classified as either back-silvered, with the silvered layer viewed through the glass. Most common household mirrors are back-silvered, since this protects the fragile reflective layer from corrosion and other damage. However, precision optical surfaces need the reflective material on the front surface of the glass to avoid introducing optical aberrations. First surface mirrors use the substrate to keep form. There are optical mirrors such as Mangin mirrors that are back-silvered as part of their optical design.
Although the silvering on a second surface mirror such as a household mirror is actual silver the "silvering" on precision optical instruments such as telescopes is aluminum. Though silver has the best initial front-surface reflectivity in the visible spectrum it is unsuitable for optical mirrors because it oxidizes and absorbs atmospheric sulfur to create a dark, low-reflectivity tarnish. Although aluminum oxidizes the thin aluminum oxide layer is transparent, so the high-reflectivity underlying aluminum stays visible; the "silvering" on infrared instruments is gold. It has the best reflectivity in the infrared spectrum, has high resistance to oxidation and corrosion. Silvering occurs in black and white photography when the black silver oxide is reduced to silver metal, the result is a reflective metallic patch in the image; the silver originates from the photosensitive silver bromide. Dielectric mirror List of telescope parts and construction Optical coating Mercury glass Mercury silvering Metallizing Tions.net, Diy mirror / mirroring / silvering