Many amusement parks and museums use hand stamps that are only visible under a special black light. Some popular Halloween costumes are accented with material that are invisible until shown under a black light. The novelty of these items has only increased over time as manufacturers find more ways to incorporate this unique property into everyday items. But exactly how do black lights work?
In short, a black light bulb produces ultraviolet light of the A variety, also known as UVA. This is different than UVB, which is more harmful to the body. Exposure of UVA rays to a substance known as phosphors creates the telltale glow that is so exotic. Phosphors are any substance that responds to radiation by emitting visible light. In essence, it converts UV radiation into visible light - hence why this substance is invisible under normal light, but glows in black light.