We know that when an element is burned, it emits glows of certain colors and no others. These very narrow wavelength intervals, unique to each element and as unmistakable as a set of fingerprints, are called emission lines . Researchers soon found that some of the emission lines exactly matched the position of solar absorption lines. Continuum lines are produced by glowing gas. Emission lines arise from electrons inside atoms in an excited state, as in a hot gas.
Absorption lines arise when atoms are in, or near, the ground state, as in a cooler gas.
Physicist Gustav Kirchhoff discovered by looking at a flame of sodium that the flame against a dark background produced emission lines for sodium. Also, the flame against a background of a brilliant ray of sunlight produced absorption lines. The sodium was absorbing the light, causing the colors associated with sodium to be missing from the beam. Absorption and emission lines occur at the same wavelength for a particular element. A gas at high pressure , a liquid, or a solid, if heated to incandescence, will glow with a continuous spectrum, or continuum.