Visible light ranges in frequency from RED (400nm) to VIOLET (700nm) Black lights emit the invisible ultraviolet light called UV-A, which has less energy than the sunburn causing UV-B light.
Some chemicals can absorb energy from invisible UV-A light and emit it as visible light. These chemicals are said to be fluorescent and can occur in everyday items such as laundry detergents, driver's licences, air mail, nail polish, oils and even money!
All materials are made up of atoms, whereby electrons move in layered orbits or 'shells' around a central nucleus of protons and neutrons (except the nucleus of Hydrogen which contains only a proton). Fluorescence occurs when the UV-A light energizes the electrons surrounding the atoms that make up the paper. As the electrons get energized, they move to the outer orbit (or electron shell). Once the electrons lose their additional energy they fall back to their original orbit, releasing any remaining extra energy as visible light.
Forensic scientists can use fluorescence of chemicals to link a crime scene to the guilty person! For more information on forensic techniques check out the crime scene investigation link.
