Fluorescent light bulb powder. Coloured scanning electron micrograph (SEM) of grains of a phosphor powder used to coat the inside of a fluorescent light bulb. It is this powder that emits the light. This is triggered by a series of events starting with an electric current passing through mercury vapour inside the fluorescent light bulb. The electric current excites the electrons in the atoms of mercury. These electrons release energy as energetic photons that hit the phosphor powder on the inside of the bulb, and the same process of electron excitation and energy loss produces safer and lower-energy photons of light that illuminate the surroundings.