A visitor left us a very interesting question to answer: 'Why do black lights make white clothing glow in the dark?' Here's the explanation. A black light gives off ultraviolet light. It looks purple, but the ultraviolet rays being emitted from the bulb are invisible. In the case of an inexpensive incandescent bulb, the rays are actually being given off by the paint on the bulb; fluorescent black lights generate UV light from the gas inside, and do a better job. Ultraviolet light is 'beyond violet' on the spectrum. It's just like visible light, except it's more energetic, and our eyes can't see it. Black lights actually produce small amounts of a type of ultraviolet light called UVA. (The kind from the sun that tans your skin is UVB). A visitor has sent us this information from the Canadian Dermatology Association: "UVA rays, while not as powerful as UVB, penetrate more deeply into the skin and are responsible for contributing to photodamage and wrinkling of the skin, premature aging, and skin cancer. UVA is prevalent in tanning parlours where tanning devices can emit two to five times more UVA than is found in natural sunlight." Some substances, like a white shirt, reflect a lot of light, and are white because they reflect all the colours in that light. Ordinary light hits them and bounces off. Substances like these can never be brighter than the light in the room around them. Other substances do  more than that. When they're hit by an energetic beam of light like UVA, they absorb the ultraviolet rays, and then  re-emit them as visible light. They turn the UVA light into light that's visible. Substances which do this are called phosphors. Anything containing phosphors will always seem brighter when illuminated by UV light because it's not only reflecting whatever room light there is, but it's also beaming out  more visible light ... that it 'made' by transforming the UV light! White shirts and socks look extra white under a black light because of the  phosphors in the detergents you washed them in. The phosphors in detergent are meant to stay in your clothing; ordinary sunlight contains some UV light as well, so your white clothing will look 'really white' because it is also emitting light. Dark clothing absorbs this extra light, so it doesn't appear to glow. New white shirts will glow because the material has been washed before being fashioned into clothing. Many other substances glow under black light, including urine, some types of paper money, and some kinds of thread and paper. They contain phosphors too. Resources Content, HTML, graphics & design by Bill Willis 2023