Is tube light/fluorescent lamp monochromatic or polychromatic? Please justify.
As you can see from the plots of spectra of different fluorescent lamps as given here, you can definitely say: yes, it's certainly polychromatic.
Moreover, true monochromatic light is an abstraction, which can't be implemented in a real device, because it's the light infinite in time - from infinite past to infinite future. It is a limit, which can be approached instead. One of the most monochromatic light source is laser. Even better one is the filtered output of certain electron transitions. Such devices are used to e.g. define unit of distance.