When the lamp is switched on, it is the mercury which converts the ultra-violet energy into visible light. This is the correct statement about the use of mercury in the manufacture of fluorescent lamps. In a fluorescent lamp, an electric current is passed through a mixture of gases that contains mercury vapor. This produces ultraviolet light, which is invisible to the human eye. However, the ultraviolet light strikes a phosphorescent coating on the inside of the lamp, causing it to emit visible light. Mercury is used in this process because it vaporizes easily and produces the ultraviolet light needed to excite the phosphorescent coating. However, because mercury is a toxic substance, it is important to dispose of fluorescent lamps properly to prevent mercury pollution in the environment.