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  The spectrum emitted by atoms is known as the “emission spectrum”. If the required energy is supplied in the form of white light, energy at specific wavelengths may be removed for excitation. These wavelengths correspond to the discrete differences in permitted energy states in the atomic shell. Accordingly, absorption lines in the form of dark lines on a continuous background appear in the spectral equipment at those wavelengths which are “swallowed” by the atom. This is the basis of AAS. Substances can only absorb light at the same wavelength as they are capable of emitting . For example , excited sodium vapour emits yellow light, but if we look through sodium vapour it seems purple since it lets through everything from the white light except yellow and it shows the colour complementary to yellow.

  The portion of white light absorbed by the sodium vapour is scattered in all directions in the form of a yellow fluorescence radiation. This is the basis AFS.

  In the same way, some substance can become fluorescent as a result of irradiation. For this it is not necessary, as in the case of sodium vapour, that only individual wavelengths should be emitted. The excited atom can return to the ground state through the radiation of a number of frequencies. The frequencies are smaller than those of exciting absorbed radiation. The fluorescence may not fade quickly- as with gases and vapours –but slowly. This kind of “ slow fluorescence” is shown by some solid material eg. calcium sulphide.