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Ultraviolet (UV) light is an electromagnetic radiation with a wavelength from 400 nm to 100 nm, shorter than that of visible light but longer than X-rays. Though usually invisible, under some conditions children and young adults can see ultraviolet down to wavelengths of about 310 nm, and people with aphakia (missing lens) can also see some UV wavelengths. Near-UV is visible to some insects and birds. UV radiation is present in sunlight, and is produced by electric arcs and specialized lights such as mercury-vapor lamps, tanning lamps, and black lights. Although lacking the energy to ionize atoms, long-wavelength ultraviolet radiation can cause chemical reactions, and causes many substances to glow or fluoresce. Consequently, biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules. Suntan, freckling and sunburn are familiar effects of over-exposure, along with higher risk of skin cancer. Living things on dry land would be severely damaged by ultraviolet radiation from the sun if most of it were not filtered out by the Earth's atmosphere, particularly the ozone layer.〔(【引用サイトリンク】title=Reference Solar Spectral Irradiance: Air Mass 1.5 )〕 More-energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has effects both beneficial and harmful to human health. == Discovery == "Ultraviolet" means "beyond violet" (from Latin ''ultra'', "beyond"), violet being the color of the highest frequencies of visible light. Ultraviolet light has a higher frequency than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted shortly thereafter, and it remained popular throughout the 19th century, although there were those who held that these were an entirely different sort of radiation from light (notably John William Draper, who named them "tithonic rays"〔"On a new Imponderable Substance and on a Class of Chemical Rays analogous to the rays of Dark Heat", J.W. Draper, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 1842, LXXX, pp.453-461〕〔"Description of the Tithonometer", J.W. Draper, The Practical Mechanic and Engineer's Magazine, January 1844, pp.122-127〕). The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, respectively.〔 〕 In 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm. In 1960, the effect of ultraviolet radiation on DNA was established.〔James Bolton, Christine Colton, ''The Ultraviolet Disinfection Handbook'', American Water Works Association, 2008 ISBN 978 1 58321 584 5, pp. 3-4〕 The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is strongly absorbed by air, was made in 1893 by the German physicist Victor Schumann.〔The ozone layer protects humans from this. 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ultraviolet」の詳細全文を読む スポンサード リンク
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