# Ångström

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The ångström or angstrom (symbol Å) (the latter spelling, without diacritics, is now usually used in English) (pronounced /ˈæŋstrəm/; Swedish: [ˈɔŋstrøm]) is an internationally recognized unit of length equal to 0.1 nanometre or 1×10−10 metres. It is named after Anders Jonas Ångström. Although accepted for use, it is not formally defined within the International System of Units(SI). (That article lists the units that are so defined.)

The ångström is often used in the natural sciences to express the sizes of atoms, lengths of chemical bonds and the wavelengths of electromagnetic radiation, and in technology for the dimensions of parts of integrated circuits. It is also commonly used in structural biology.

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### History

The ångström is named after the Swedish physicist Anders Jonas Ångström (1814–1874), one of the pioneers in the field of spectroscopy, who 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 millimetre, or 1×10−10 metres. That unit of length became known as the Ångström unit, and later simply as the ångström.

The human eye is sensitive to wavelengths from about 4,000 ångströms (violet) to 7,000 ångströms (red) so the use of the ångström as a unit provided a fair amount of discrimination without resort to fractional numbers. As it is close to the scale of atomic and molecular structures, it also became popular in chemistry and crystallography.

Although intended to correspond to 1×10−10 metres, 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 metre, but in 1960, the metre itself was redefined in spectroscopic terms, thus aligning the ångström as a submultiple of the metre. In short, one nanometre is equal to ten ångströms.