The emission of light from a material when subject to photons of another wavelength. A fluorescent object often emits visible light when it receives ultraviolet light. For example, fluorescent lights are glass tubes containing mercury in a partial vacuum, and the tubes are lined with a fluorescent phosphor. When the mercury is excited by electricity, it emits ultraviolet (UV) light, which causes the phosphor to emit visible light. Fluorescence also occurs in nature; for example, fireflies and certain deep sea fish have fluorescent qualities.