Ultraviolet light is a type of electromagnetic radiation that makes black-light posters glow, and is responsible for summer tans and sunburns. The UV range of the spectrum is characterized by wavelengths between 100 and 400 nanometers (nm). It includes the long-wave UV-A (315 to 400 nm), which causes suntan (or burn), medium-wave UV-B, (280 to 315 nm) used to treat skin conditions such as psoriasis, and short-wave UV-C (100 to 280 nm).