You have probably seen black lights at amusement parts, science museums and Halloween displays. Black light makes fluorescent colors "glow in the dark." For example, if you have a fluorescent poster and shine a black light on it in a dark room, the poster will glow brightly. You may have also seen pieces of paper that look blank in regular light but spell out a glowing message under a black light. Many amusement parks use hand-stamps that are invisible until you view them under black light.
So what is going on here? A black light bulb produces "black light." If you turn on a black light bulb in a dark room, what you can see from the bulb is a purplish glow. What you cannot see is the ultraviolet light that the bulb is also producing.
Our eyes can see visible light in a spectrum ranging from red through orange, yellow, green, blue and violet. Above violet is ultraviolet light, which we cannot see. How Sun Tans and Sunburns Work discusses ultraviolet light and its effects on our skin. A black light bulb produces UVA light (as opposed to UVB light, which is much more harmful).
What you see glowing under a black light, whether it is a fluorescent poster or an invisible hand stamp or a newly-washed white T-shirt, is phosphors.
A phosphor, as described in How Television Works, is any substance that emits visible light in response to some sort of radiation. In other words, a phosphor converts the energy in the radiation into visible light.
So a fluorescent paint on a fluorescent poster contains a phosphor that converts UV radiation into a specific color of visible light. Normal colors simply reflect light, but a fluorescent color absorbs the radiation and re-emits it in the visible spectrum (sort of like a light bulb emits light), so it looks much brighter than a normal color. White T-shirts and socks normally glow under a black light because modern detergents contain phosphors that convert UV light into white light. This makes whites look "whiter than white" in normal sunlight. What you are seeing in sunlight is the normal reflection of visible white light from the cloth, as well as the emission of white light that the phosphors create from UV light in sunlight. The T-shirt really is whiter than white!
Many other substances fluoresce naturally under black light, including liquid detergents (as we already talked about), quinine (in tonic water), urine, and some types of paper money.
Here are some interesting links: