Black lights look just like normal fluorescent lamps or incandescent light bulbs, but they do something completely different. Switch one on, and white clothes, teeth and various other things glow in the dark, while the bulb itself only emits faint purple light. These devices are all around us -- in clubs, science museums, amusement parks and teenagers' bedrooms, among other places -- but to most people, they're a total mystery. What Glows? If you walked around all night with a portable black light, you would discover that there are phosphors all over the place. There are lots of natural phosphors, in your teeth and fingernails, among other things. There also a lot of phosphors in manmade material, including television screens and some paints, fabric and plastics. Most fluorescent colored things, such as highlighters, contain phosphors, and you'll find them in all glow-in-the-dark products. Clubs and amusement parks use special black light paint that glows different colors. You can also buy fluorescent black light bubbles, invisible black light ink, fluorescent black light carpet and even fluorescent black light hair gel. The first thing most people notice when you switch on a black light is that some of their clothing glows. This is because most laundry detergents contain phosphors to make whites appear brighter in sunlight. Sunlight contains UV light that makes the whites glow "brighter than white." Dark clothes don't glow because the dark pigments absorb the UV light. In addition to making people and fluorescent posters look cool, black lights have some practical applications. For example: " Appraisers use them to detect forgeries of antiques. Many paints today contain phosphors that will glow under a black light, while most older paints do not contain phosphors. " Repairmen use them to find invisible leaks in machinery -- they inject a little fluorescent dye into the fuel supply and illuminate it with a black light. For example, they might detect an invisible air conditioner leak by adding fluorescent dye to the refrigerant. " Law enforcement officers can use them to identify counterfeit money. The United States and many other countries include an invisible fluorescent strip in their larger bills that only shows up under a black light. " Amusement parks and clubs use them to identify invisible fluorescent hand stamps for readmission. " Forensic scientists use them to analyze crime scenes. To pick out fingerprints, for example, they often dust with fluorescent dye under a black light. This makes it easier to pick the fingerprints out from surrounding dirt. Black lights can also identify semen and other bodily fluids that naturally fluoresce. Most of these uses, as well as dozens of others, follow a common theme -- the black lights make the invisible visible or isolate one specific substance from everything around it. When you think about it, there are dozens of situations where you could put this phenomenon to work. The applications are potentially endless!