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UV light, short for Ultraviolet Light, is a type of light energy making up one part of the electromagnetic spectrum, which spectrum includes gamma and x-rays, UV light, visible light, infrared rays, microwaves, and radio waves, listed in order of decreasing frequency and increasing wavelength. UV light thus has a wavelength shorter than that of visible light, and can not be detected by the human eye. While UV light itself is invisible, it causes many substances to glow or fluoresce in a variety of colors visible to the human eye. This ability to make the invisible visible makes UV lamps valuable for mineral prospecting, criminal investigations, postal stamp evaluation, etc.
UV light is divided, at a minimum, into both Shortwave and Longwave radiation. Shortwave UV is produced by low pressure mercury arcs, with a wavelength of 254 nanometers, while Longwave, produced by low to high pressure mercury arcs, has a wavelength of from 320 to 400 nanometers. A nanometer is a unit of length used to define wavelengths of energies in the electromagnetic spectrum, equaling one millionth of a millimeter.
While Longwave UV, sometimes referred to as "blacklight", is safe, Shortwave UV can irritate the eyes and burn the skin, so exposure should be controlled and ideally, UV goggles should be worn to prevent eye damage from inadvertently looking at the light source.
As before stated, UV light directed at certain materials causes fluorescence, originally named after the mineral Fluorite for its blue glow under UV. Phosphorescence is a type of fluorescence that continues even after the UV light is removed. It can last from fractions of a second to weeks. Of the fluorescent minerals, about 80 to 90 % of them fluoresce brighter under Shortwave than under Longwave UV.