" Different kinds of gases (e.g., gases based on different chemical elements) absorb different spectral colors. So if one has a gas of unknown composition, one can deduce what it's made of by seeing what light it absorbs! In the language of our generalized chemistry, the message of Fraunhofer's dark lines, as interpreted by Bunsen and Kirchhoff, is that a given atom of substance will combine with-that is, absorb-only specific elements of light-that is, spectral colors-while ignoring others. There is also a converse effect, that heated gas will emit light in preferential colors, creating bright lines in the spectrum. Altogether, these dark and bright lines are like fingerprints identifying the responsible substances. "

― Frank Wilczek , A Beautiful Question: Finding Nature's Deep Design