Mercury gas atoms in fluorescent bulbs are bombarded by electrons emitted from electrodes at the ends of the tube.
This causes electrons in the atoms to jump to higher energy levels and, when the electrons revert to lower energy levels, UV photons are released. These "invisible" UV
photons are converted to visible light by coating the inner surface of the tube with a phosphor that absorbs UV photons and converts them to lower energy visible
light (the "rainbow" background). Visible lines produced directly by Mercury atoms are also seen
(the emission lines in the yellow, green and violet).
The "cool white" versions of these bulbs are deficient in red so things such as skin color don't look very appealing. Some bulbs (called
"natural white" use different phosphors to produce a more "natural" looking illumination.
Spectrum of Cool White Fluorescent Bulb