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"Black" lights are really purple, shedding a form of ultraviolet light that lends an eerie glow to a dark room. They are perhaps best known as a party accessory of the 1960s and 1970s, but are extensively used in non-destructive testing of everything from crime scenes to banknotes.
The fun quality of black lights is related to their ability to light up anything phosphorescent -- including people's teeth and fingernails. Clubs and amusement parks use phosphorescent paint to achieve special effects when black lights are lit. For a fun experiment, try writing with a highlighter, then shining a black light on it!