Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays. Wavelengths of UV radiation range from 5 to 400 nm, which may be further subdivided into the UV-A, UV-B, and UV-C ranges. UV radiation contains about 9% of the total energy of the solar electromagnetic spectrum. Such radiation has marked actinic and bactericidal action, and produces fluorescence in a number of substances. Ultraviolet radiation from the sun is responsible for many complex photochemical reactions characteristic of the upper atmosphere, for example, the formation of the ozone layer through ultraviolet dissociation of oxygen molecules followed by recombination to form ozone. The absorption of UV by stratospheric ozone and upper atmospheric oxygen is sufficiently strong that very little ultraviolet radiation with wavelengths shorter than about 300 nm reaches the earth's surface.