Fluorescent lights work by running electricity through a tube full of mercury vapor.

  1. Electrodes are found at each end of the tube, and the flow of electrons across the tube excites the Mercury atoms.
  2. The electrons of the mercury atoms are raised one energy level.
  3. In the return of the electrons the ground state, the atoms emit a 254 nanometer photon, which is in the ultraviolet range.
  4. The ultraviolet light then hits a powder of fluorescent phosphors coating the inside of the tube.
  5. The phosphors absorb the photons, and their electrons are raised to higher energy levels.
  6. When these electrons return to the ground state, various wavelengths of visible light are then emitted (varying by the composition of the phosphors).

Some of the energy here is lost as heat, but 90% of the energy expended in the operation of a fluorescent bulb is emitted as light. Most modern fluorescent lights in the US oscillate at 120 Hz, in time with the oscillations of the electric current.

Within a fluorescent light, the mercury gas within the tube that emits ultraviolet photons when excited by electricity, and the phosphor crystals coating the inside of the tube that fluoresce when hit by that ultraviolet light. Mercury ain't no good for you at all, but it's found in extremely small quantities in fluorescent bulbs—far less than 0.1 milligram—that it poses little danger. The phosphors found inside the bulb are completely harmless.

The composition of the phosphors varies by the exact shade of light desired (e.g. cool white, warm white, full spectrum, &c.), but can include any of the following:

The phosphor crystals are laced with certain impurities that cause them to emit light in the visible range of the spectrum, and these impurities may include the following: