Depends on the type of bulb - the details are slightly different. Incandescent bulbs heat a filament to bright white glow. Neon, fluorescent and other gas type bulbs excite particles of the gas to cause them to glow. Mercy 'vapor' types heat up mercury (or other elemental materials) and 'excite' the gas from that causing it to 'glow'. More details on each type can be found at these links: Incandescent type: http://home.howstuffworks.com/light-bulb.htm Fluorescent type: http://www.goodmart.com/facts/light_bulbs/compact_fluorescent_diagram.aspx Other Gas type bulbs: http://www.sciam.com/article.cfm?id=how-do-neon-lights-work

1) "The incandescent light bulb or incandescent lamp is a source of artificial light that works by incandescence (a general term for heat-driven light emissions which includes the simple case of black body radiation). An electric current passes through a thin filament, heating it until it produces light. The enclosing glass bulb prevents the oxygen in air from reaching the hot filament, which otherwise would be destroyed rapidly by oxidation." "Incandescent light bulbs are gradually being replaced in many applications by (compact) fluorescent lights, high-intensity discharge lamps, LEDs, and other devices, which produce the same amount of visible light but use less electrical energy. Some jurisdictions have or are considering banning the sale of incandescent lightbulbs in favour of more energy-efficient lighting." Source and further information: http://en.wikipedia.org/wiki/Incandescent_light_bulb Further information - how to change a light bulb (jokes): http://en.wikipedia.org/wiki/Lightbulb_joke 2) "A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in a plasma that produces short-wave ultraviolet light. This light then causes a phosphor to fluoresce, producing visible light. Unlike incandescent lamps, fluorescent lamps always require a ballast to regulate the flow of power through the lamp." Source and further information: http://en.wikipedia.org/wiki/Fluorescent_light