A black light
is a type of light that emits ultraviolet light. Since ultraviolet light isn't visible to our eyes, most of the light is invisible or "black" to us. Usually the type of black lights you or I would buy give off enough visible violet light that you can tell when the light is on. Ultraviolet light are very energetic, so when the light strikes some materials they absorb the radiation and re-release it in the visible spectrum. This is how fluorescence and phosphorescence work. These processes make materials appear to glow in the dark because there is a time delay between the absorption of the black light and the release of the visible light. Fluorescence occurs so quickly you see a material exposed to black light glow when the light is on and stop glowing when the light is turned off. Phosphorescence is delayed, so black light absorbed by phosphorescent objects can release light several seconds, minutes or even hours after exposure.
Have you ever wondered what glows under a black light? You can carry around a black light penlight to test various objects in your home or in stores. If you don't have access to a black light, here are some lists of things that glow under a black light and a photo gallery to show you what they look like.
Video - What Glows under a Black Light?
Black Light Photo Gallery
Black Light Projects
List of Materials That Glow under a Black Light