Flame emission spectra of the alkali metals, as recorded by Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887). Working together, Bunsen and Kirchhoff were the first to observe that heated elements emitted a light of characteristic wavelength. When the elements are heated the electrons within it become excited. To return to their ground state they release the extra energy as light radiation at a specific wavelength. Image taken from Lessons in Elementary Chemistry (Roscoe, 1873).