A mercury discharge tube produces numerous strong spectral bands. The spectrum of the mercury-filled fluorescent lights in the lab shows three strong bands (one red, one green, and one blue). Why?

see below

Explanation:

Atomic Emission Spectrum result from electron transitions. Electrons can adsorb and release energy while residing within the electronic volume of the atom. According to the Bohr Model of the atom, when an electron adsorbs electromagnetic energy (light) it moves to a higher principle orbital level and then falls back toward the original orbital level. The electron on returning toward the original orbital level gives up EMR as it moves through the lower energy levels. However, the electron may stop at any of the lower energy levels giving off 'discrete' amounts of energy in the fall. Electron transistions that end on principle energy levels of n = 3 or higher give off Infrared EMR and those that end on the original ground state energy level (n = 1) give off ultraviolet EMR.  However, transitions that end on n = 2 give off 'Visible EMR' that can be observed through a spectroscope.  Discrete transitions that occur generating the blue spec band is an n=5 => n=2 transition, the green spec band is an n=4 to n=2 transition and the red spec band an n=3 to n=2 transition.

One can verify the transition energy using the equation ΔE = hc/λ where h = Plancks Constant = 6.63 x 10⁻³⁴ j·sec, c = speed of light = 3 x 10⁸ m/s and λ = wavelength in meters.  Recommend searching Hydrogen Emmission Spectra and note electron transition diagrams for electrons.  

they are all solids.