Ultraviolet Radiation, electromagnetic radiation that has wavelengths in the range between 400 nm, the wavelength of violet light, and 15 nm, the length of X rays. (The nanometre, nm, equals a millionth of a millimetre, or 40 billionths of an inch.) Ultraviolet radiation is produced artificially by electric arc lamps. Natural ultraviolet radiation is produced principally by the Sun.
Ultraviolet radiation can be harmful to living things, particularly when the wavelengths are small. Ultraviolet radiation with wavelengths below 300 nm is used to sterilize surfaces because it kills bacteria and viruses. In humans, exposure to ultraviolet radiation of wavelengths under 310 nm can cause sunburn; prolonged exposure over many years can cause skin cancer.
The Earth's atmosphere protects living organisms from the Sun's ultraviolet radiation. If all the ultraviolet radiation produced by the Sun were allowed to reach the surface of the Earth, most life on Earth would probably be destroyed. Fortunately, the ozone layer of the atmosphere absorbs almost all of the short-wavelength ultraviolet radiation, and much of the long-wavelength ultraviolet radiation. However, ultraviolet radiation is not entirely harmful; a large part of the vitamin D that humans beings and animals need for good health is produced when the skin is irradiated by ultraviolet rays.
When exposed to ultraviolet light, many substances behave differently than when exposed to visible light. For example, when exposed to ultraviolet radiation, certain minerals, dyes, vitamins, natural oils, and other products become fluorescent—that is, they appear to glow. Molecules in the substances absorb the invisible ultraviolet light and are energized, then shed their excess energy by emitting visible light. See Luminescence. As another example, ordinary window glass, transparent to visible light, is opaque to a large range of ultraviolet rays, particularly those with short wavelengths. Special-formula glass is transparent to the longer ultraviolet wavelengths, and quartz is transparent to the entire naturally occurring range.
Ultraviolet astronomy has been carried out since the early 1960s with the aid of detectors on artificial satellites, providing data on stellar objects that cannot be obtained from the Earth's surface. An example of such a satellite is the International Ultraviolet Explorer, launched in 1978.