ANGSTROM

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The unit of wavelength of light is - Angstrom.

The angstrom or ångström  is a unit of length equal to 10⁻¹⁰ m (one ten-billionth of a meter) or 0.1 nm. Its symbol is the Swedish letter Å.

The ångström is often used in the natural sciences and technology to express the sizes of atoms, molecules, and microscopic biological structures, the lengths of chemical bonds, the arrangement of atoms in crystals, the wavelengths of electromagnetic radiation, and the dimensions of integrated circuit parts. Atoms of phosphorus, sulfur, and chlorine are 1 Å in covalent radius, while a hydrogen atom is 0.25 Å; seeatomic radius.

The unit was named after the Swedish physicist Anders Jonas Ångström (1814–1874). The symbol is always written with a ring diacritic, as in the Swedish letter. Although the unit's name is often written in English without the diacritics, the official definitions contain diacritics.

History

Anders Jonas Ångström was one of the pioneers in the field of spectroscopy, and is known also for studies of astrophysics, heat transfer, terrestrial magnetism, and the aurora borealis.

In 1868, Ångström created a chart of the spectrum of solar radiation that expressed the wavelengths of electromagnetic radiation in the electromagnetic spectrum in multiples of one ten-millionth of a millimeter (or 10⁻⁷ mm.) Since the human eye is sensitive to wavelengths from about 4,000 to 7,000 Å, what we commonly call visible light, that choice of unit allowed sufficiently accurate measurements of visible wavelengths without resorting to fractional numbers.^{[citation needed]} The unit then spread to other sciences that deal with atomic-scale structures.

Although intended to correspond to 10⁻¹⁰ meters, for precise spectral analysis the ångström needed to be defined more accurately than the metre which until 1960 was still defined based on the length of a bar of metal held in Paris. In 1907, the International Astronomical Union defined the international ångström by declaring the wavelength of the red line of cadmium in air equal to 6438.46963 international ångströms, and this definition was endorsed by the International Bureau of Weights and Measures in 1927. From 1927 to 1960, the ångström remained a secondary unit of length for use in spectroscopy, defined separately from the meter. In 1960, the meter itself was redefined in spectroscopic terms, and then the ångström was redefined as being exactly 0.1 nanometers.

Although internationally recognized, the ångström is not formally a part of the International System of Units (SI); the closest SI unit is the nanometre (10⁻⁹ m). Its use is officially discouraged by the International Committee for Weights and Measures and is not included in the European Union's catalogue of units of measure that may be used within its Internal Market.

Symbol         

Unicode includes the formal symbol at U+212B Å angstrom sign . However, the ångström sign is also normalized into U+00C5 Å latin capital letter a with ring above.