The light whose wavelength (about 380 nanometers) is just long enough not to be x-rays, but just enough shorter than violet light so that it is not visible to the human eye. Ultraviolet is also known by the short form of "UV." It is sometimes called black-light because ultraviolet lamps (usually a mercury-vapour lamp) appear quite dark even when lit, and because of the peculiar way it illuminates certain kinds of surfaces, such as day-glo colours. (Artlex.com)

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