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Concept
Magnesium fluoride
Summary
Magnesium fluoride is an inorganic compound with the formula . The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics that are also used in space telescopes. It occurs naturally as the rare mineral sellaite.
Magnesium fluoride is prepared from magnesium oxide with sources of hydrogen fluoride such as ammonium bifluoride:
Related metathesis reactions are also feasible:
The compound crystallizes as tetragonal birefringent crystals. The structure of the magnesium fluoride is similar to that of rutile, featuring octahedral cations and 3-coordinate anions.
In the gas phase, monomeric molecules adopt a linear molecular geometry.
Magnesium fluoride is transparent over an extremely wide range of wavelengths. Windows, lenses, and prisms made of this material can be used over the entire range of wavelengths from 0.120 μm (vacuum ultraviolet) to 8.0 μm (infrared). High-quality, synthetic magnesium fluoride is one of two materials (the other being lithium fluoride) that will transmit in the vacuum ultraviolet range at 121 nm (Lyman alpha). Lower-grade magnesium fluoride is inferior to calcium fluoride in the infrared range.
Magnesium fluoride is tough and polishes well but is slightly birefringent and should therefore be cut with the optic axis perpendicular to the plane of the window or lens. Due to its suitable refractive index of 1.37, magnesium fluoride is commonly applied in thin layers to the surfaces of optical elements as an inexpensive anti-reflective coating. Its Verdet constant is 0.00810 arcmin⋅G–1⋅cm–1 at 632.8 nm.
Chronic exposure to magnesium fluoride may affect the skeleton, kidneys, central nervous system, respiratory system, eyes and skin, and may cause or aggravate attacks of asthma.
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Anti-reflective coating
An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the efficiency since less light is lost due to reflection. In complex systems such as cameras, binoculars, telescopes, and microscopes the reduction in reflections also improves the contrast of the image by elimination of stray light. This is especially important in planetary astronomy.
Prism (optics)
An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the wavelengths for which they are designed.
Optical coating
An optical coating is one or more thin layers of material deposited on an optical component such as a lens, prism or mirror, which alters the way in which the optic reflects and transmits light. These coatings have become a key technology in the field of optics. One type of optical coating is an anti-reflective coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacle and camera lenses. Another type is the high-reflector coating, which can be used to produce mirrors that reflect greater than 99.