Fluorescent tubes are filled with low pressure mercury, when an electric current is passed through, the electrons in mercury are excited and move to a high energy level, this high level state is unstable and so the electron moves back to its original state, but on doing so, it emits an electromagnetic wave with energy equivalent the difference in energy level. This is UV light and the electrons in the phosphor coating inside the fluorescent tube are excited, and releases visible light when the electrons return to its orginal energy state, which provides the glow in fluorescent lights.