A black light emits ultra violet light waves at the very end of the visible spectrum. The UVA light from a black light has longer wave lengths than most visible light. Don't tan under a black light as these longer waves penetrate further into the skin and may produce hydroxyl and oxygen radicals, aka skin cancer. It would take a lot of time under a black light to get cancer. Sir Isaac Newton first explained the idea of a light spectrum in his book Opticks written in the 18th Century.
The great thing about a black light is looking at different objects. Anything white will appear to glow. Tonic water will glow blue under a black light. The quinine in tonic water reflects the UVA light. Many bodily fluids contain florescent materials and will also glow under a black light. Forensic scientists use black light to detect blood at crime scenes. If you crush a Vitamin B-12 tablet and dissolve it in vinegar it will glow yellow. An empire scorpion will glow blue-green under a black light. Maybe a blacklight is a good survival tool for the desert? Canola oil will also glow under a black light. Chlorophyll will glow red.
Do you have a black light handy? What have you found that glows in UVA light?