Some of the things that glow under a black light include white paper, tonic water, petroleum jelly, chlorophyll, money, postage stamps, jellyfish and some minerals. These substances glow because they absorb the ultraviolet light given off by the black light and then re-emit it.
Black lights work off of the property of fluorescence, which means that objects absorb light with a higher energy and emit light of a lower energy. While the ultraviolet light the black light gives off is invisible to the human eye, it is still absorbed by the objects near the source of the light and then reflected through them. Humans typically see light that ranges from red to blue. Both infra red and ultraviolet light cannot be detected by looking at them directly. However, some chemicals, materials, and other objects absorb ultraviolet light better than others, making them appear to glow under a black light.
The principal of fluorescence is also used in science. Crime scene investigators use fluorescent light to investigate car accidents and other crimes to find evidence invisible to the naked eye. Because antifreeze glows under a black light, investigators can use leaked antifreeze to track the movements of a car on a road. Scientists also use fluorescent light to study organisms and cells.