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Ultraviolet
Radiation, electromagnetic
radiation that has wavelengths in the range between 4000 angstrom
units (Å), the wavelength of violet light, and 150 Å, the length of X
rays. Natural ultraviolet radiation is produced principally by the sun.
Ultraviolet radiation is produced artificially by electric-arc lamps .
Ultraviolet
radiation is often divided into three categories based on wavelength,
UV-A, UV-B, and UV-C. In general shorter wavelengths of ultraviolet
radiation are more dangerous to living organisms. UV-A has a wavelength
from 4000 Å to about 3150 Å. UV-B occurs at wavelengths from about 3150
Å to about 2800 Å and causes sunburn; prolonged exposure to UV-B over
many years can cause skin cancer. UV-C has wavelengths of about 2800 Å to
150 Å and is used to sterilize surfaces because it kills bacteria and
viruses.
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
portion of the vitamin D that humans and animals need for good health is
produced when the human's or animal's 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, become
energetic, then shed their excess energy by emitting visible light. As
another example, ordinary window glass, transparent to visible light, is
opaque to a large portion of ultraviolet rays, particularly ultraviolet
rays with short wavelengths. Special-formula glass is transparent to the
longer ultraviolet wavelengths, and quartz is transparent to the entire
naturally occurring range.
In
astronomy,
ultraviolet-radiation detectors have been used since the early 1960s 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.
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