Based on the spectrum of the fluorescent lamp, what is the main difference between the radiation emitted by a heated solid (filament) and the radiation emitted by an atomic gas (mercury gas in the fluorescent tube)?

Answer 1

The main difference is that a heated filament gives rise to a continuum while the Hg gas gives rise to a line spectrum. You don't see a line spectrum, of course, because the Hg line spectrum shines on the coating on the inside walls of the fluorescent tube and you see the light being emitted by that coating.