Fluorescence occurs when an electron in an atomic orbital absorbs energy from some source (like an interaction with a photon, or a collision with another atom) is promoted to a higher energy level and then, on 'falling back', releases some of that energy in the form of visible light.The missing energy is usually converted to thermal energy (infra red transitions), making the tube slightly hot.
A fluorescent light is
a type of gas discharge tube,
is made of glass and is narrow, with two electrical connections on each of the metal caps that seal the ends of the tube.
contains an inert gas (such as argon, neon, or krypton) and mercury vapour.
a tube with gases inside having a pressure of about 0.003 atmospheres.
The mixture of mercury and gas is not conductive when the tube is off.
How it works:
A high voltage discharge is needed to start the flow of current. The initial high voltage burst is necessary to produce electrons with enough kinetic eergy to ionise the mercury atoms - the atoms in the gas need to be sparse enough (low pressure) so that KE can be built up to a level that will allow ionisation of the mercury atoms.
The high speed electrons cause the mercury atoms to ionize. After this has taken place, the voltage can lowered - as the initial ionisations produce more of the same. The voltage required ranges from 100 volts for tubes under 30 watts and 100 to 175 volts for tubes of 30 watts or more.
A pair of electrode filaments are located at the metal ends of the tube. They produce electrons that ionise the mercury atoms. They do this by thermionic emission. The filaments remain hot when the tube is lit producing a continuous electrical discharge.
The flow of electrons through the gases excites the electrons in the mercury atoms, which then emit ultraviolet (UV) radiation.
There is a good reason why the lamp contains only a small amount of mercury, which must be vaporized to support the lamp current and generate light. At low temperatures the mercury is in the form of dispersed liquid droplets. As the lamp warms, more of the mercury is in vapour form - therefore more can be ionised by the electrons passing through it BUT at higher temperatures, self-absorption in the vapour reduces the yield of UV and visible light - so you cannot have too much mercury in the vapour form otherwise it will not produce light. Since mercury condenses at the coolest spot in the lamp, careful design is required to maintain that spot at the optimum temperature, around 40 °C.
The inside of the tube is coated with a phosphorous material that emits visible light when excited with UV and the tube gives off light. By carefully selecting the fluorescent powders (called "phosphors"), the manufacturer of the light can tailor the light's coloration. Phosphors in CF bulbs are a mix to produce excellent color rendering and warmth similar to incandescent bulbs. Other phosphor mixtures are warm white, cool white, deluxe warm white, and deluxe cool white.
Advantages over the filament light bulb
Fluorescent bulbs are four to six times more efficient than incandescent bulbs.
The lamp allows brightly lit workplaces to remain
at a cool temperature due to its greatly increased efficiency.
More detailed background reading