When atoms in a gaseous state are given energy by either heating or passing an electric current through them electrons move to a higher energy level.They emit photons when they fall back to lower energy levels.The photons will be of specific frequency which will be characteristic to the element. The frequencies emitted correspond to the difference in energy between the energy levels of the electrons (E = hf). A line spectrum of coloured lines against a black background will be produced (emission spectrum).The fact that only certain frequencies are emitted rather than a continuous spectrum is the basis for quantum theory whereby electrons can only have one of a discrete set of energy values.