can somebody briefly explain why spectral lines are useful in determining the chemical composition of ..?

Question

.. their source??

Answer 1

Each chemical or for that matter element has its own specific signature like a finger print When heated it emits a specific wave length of light in the visible or invisible spectrum. So when I see a line in a specific position on the spectrum I know that it is Hydrogen or H2o Or ethyl alcohol. This is over simplified but basically correct.