Atomic Absorption:

Atoms in the gaseous state could absorb ultraviolet or visible radiation or x-rays and result within a spectrum consisting of a number of very sharp absorption lines, features of the atomic species. This is known as a line spectrum. Let us consider the easy case of absorption through gaseous sodium atom. It exhibits two sharp absorption peaks within the yellow region of visible spectrum (589 nm and 589.6 nm) as display in Figure, consequent on excitation of 3s electron to two of the 3p states.  Absorption in the UV region instead results in signals at 330 nm and 285 nm because of the excitation of 3s election to 4p and 5p orbital respectively.

Figure: Absorption spectrum of sodium vapours in the visible range

Atomic absorption spectra of alkali and alkaline earth metals are simpler.  On the other hand the transition elements give rise to multitudes of lines, yet each element has its own characteristic absorption lines and is useful for qualitative and quantitative studies.