A blacklight is a special lamp that gives off mostly invisible ultraviolet light, which makes certain materials glow with bright, unusual colors in the dark. When you shine a blacklight on a white t-shirt, it might suddenly glow brilliant purple or blue. Tonic water glows bright blue under a blacklight. Some minerals and scorpions become visible under a blacklight that you'd never see in normal light.

The name seems backward at first: why call it a black light when it makes things glow? The answer is that the ultraviolet rays it produces can look dark purple or almost black to our eyes, even though they're actually light waves. These special rays hit certain substances and cause them to fluoresce, releasing visible light energy that creates that eerie glow.

Scientists use blacklights to study everything from minerals to ancient manuscripts. Museums use them to authenticate paintings and detect forgeries. Crime scene investigators use them to find evidence invisible to the naked eye. Amusement parks create glowing effects for rides and haunted houses. Some people even use blacklight posters in their rooms because the fluorescent colors look so vivid and dramatic when the regular lights go out.