Black lights use UV (ultraviolet) light, and can be used to create some of the coolest special effects you've ever seen. Almost anything white will glow when you hit them with light from blacklights. When used to illuminate a black light poster or scenery or walls painted with UV reactive paints the colors become more vivid and even "neon" looking. You can even paint one scene to be visible with regular light and another to only be seen under the UV black light. Blacklights are perfect for any party, dorm room or club. There's no limit to the fun you can have under black light!
Blacklights make phosphors glow, and only objects with phosphor will appear to be illuminated. A white T-shirt or white socks may glow, not because they're white, but because they were laundered with detergent containing phosphors - fluorescent powders that, if you expose them to ultraviolet light, absorb the light energy and immediately re-emit some of it as visible light. This process is called fluorescence. Many detergents now have little or no phosphor in them so the white shirts remain dark or, at best, dimly lit.
Black lights have hundreds of uses. In addition to just plain having fun with them, they can be used for checking hand stamps, finding pet stains (such as cat urine), checking for bacteria, applying black light sensitive cosmetics, searching for antique glass. Professional appraisers use black lights to assess the condition of antiques and to certify authenticity. This powerful ultraviolet light exposes paint, glue, and other repairs hidden to the naked eye.
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. For example, they might detect an invisible air conditioner leak by adding fluorescent dye to the refrigerant. They can be used 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.
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