A fluorescent light uses a lining to emit visible light, explain why this is necessary and how it works.

The vapour inside a fluorescent tube is excited and ionised by the electrode either end of the tubes. The excited electrons inside of the vapour are unstable and so decay into lower energy states. The difference in the energy states it decays to is equal to the energy of the photons released. This difference in the vapour is too high and so the vapour does not emit visible light. This is where the lining comes in. The lining uses the photons from the vapour to excite its electrons and forces them into higher energy states. They then decay, but due to the energy states being closer together, when they de-excite, they emit photons of visible light, as they have the specific energy level profile to emit visible light.