Emission of radiation:

Electrons in the excited state are extremely unstable and move back down to a ground state or a lower energy state quite rapidly. As they do so, they emit the energy within the form of radiation of features wavelength that is measured through a detector. For a few metals this radiation corresponds to wavelengths of light within the visible region of the electromagnetic spectrum and is observed as a features color of the flame. Since electrons from different energy levels are able to emit light as they relax, a flame color observed will be a combination of all the variant wavelengths emitted through the different electrons within the metal atom under investigation.

The flame emission spectrum of the element sodium could be acquired through placing a solution of sodium chloride within an appropriate flame. At the temperature of the flame (2000ºC to 3000 ºC) the outer electron of the sodium atom is promoted from the ground state 3s orbital to excited porbitals (3p, 4p, 5p). The relaxation of the excited electron provides rise to the characteristic emission spectrum of the sodium atom. An emission spectrum of the sodium atom is relatively simple and consists of about 40 signals or lines; the most prominent emission signals being within the region of 285 nm, 330 nm and 590 nm.  Figure displays a portion of the flame emission spectrum for sodium atom; the excitation being complete through an oxyhydrogen flame.