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Ultraviolet light is invisible, but black lights or UV-lamps also emit some visible violet light.

Ultraviolet light is invisible, but black lights or UV-lamps also emit some visible violet light.

Kallemax, wikipedia.org
Many fluorescent and phosphorescent materials absorb and re-emit the energy from an ultraviolet lamp, sometimes called a 'black light' because the ultraviolet light is not visible.
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