Compact fluorescent lights (CFLs) and other fluorescent light bulbs contain gases (argon and mercury vapor) that produce invisible ultraviolet (UV) light when stimulated by electricity. When the UV light hits the white phosphor coating inside the fluorescent bulb, the phosphor illuminates or “fluoresces,” changing the UV light into visible light. CFLs are very energy-efficient, using only about one-fifth the energy of a standard incandescent bulb. This is because all of the electricity they use goes toward creating light, whereas the energy used by standard incandescent bulbs creates heat as well as light.