Posters coated with phosphors glow under black lights.
A black light is a fluorescent light bulb that, when lit, looks purple. Black lights make certain objects in an otherwise dark space seem to glow.
Construction
A regular fluorescent bulb is coated with a thin phosphorous coating that absorbs ultraviolet (UV) light and emits visible light. Black lights use a different phosphor coating that works in much the same way; however, the coating allows a small amount of UVA (long-range ultraviolet) light, as well as a small amount of violet and blue visible light, through.
Glowing
Any item that glows under a black light contains phosphors. A phosphor is a natural or synthetic substance that puts off a visible light when exposed to radiation, such as UVA light. When the UVA light from a black light hits the phosphor, the phosphor glows.
Phosphor Items
Most fluorescent or glow-in-the-dark items contain phosphors, as do many laundry detergents and household cleaners. White paper, white clothing, petroleum jelly, tonic water, teeth, some energy drinks and certain plants glow under black lights, as do body fluids such as urine or saliva.
