ultraviolet (UV) radiation, a range of electromagnetic waves extending from the violet, or short-wavelength, end of the visible spectrum to the beginning of the x-ray spectrum. Near-UV radiation covers a range of wavelengths from 400 to 320 nm, middle-UV from 320 to 280 nm, and far-UV from 280 to about 10 nm. About 5% of the energy from the sun consists of UV radiation, but little reaches the earth because much is absorbed by oxygen and ozone in the atmosphere. Window glass also absorbs UV radiation. Artificial sources of UV radiation include the iron arc, the carbon arc, and the mercury vapor arc. For maximum transmission, prisms and lenses used for work in the UV region must be made of quartz, fluorite, or synthetic halides, which are transparent to UV radiation. In medicine, ultraviolet radiation is used in the treatment of rickets and certain skin conditions. Milk and some other foods become activated with vitamin D when exposed to this type of energy. UV radiation also causes certain substances to fluoresce or phosphoresce, a useful characteristic in such diverse applications as lighting and the identification of minerals.
