... arises when light is passed through a sample of gas that is not so hot and therefore the atoms have less energy. When this was done with sodium (as in the emission spectrum) dark lines were found which corresponded exactly with the yellow lines emitted by very hot sodium. This showed that anything that absorbs also emits. Below is a diagram of the absorption spectrum. (Ref3) The difference between the emission and absorption spectrum is that in an emission spectrum the wavelengths of light that are absorbed from a blackbody will emit exactly those colours as the gas de-excites. Whereas in the absorption spectrum the colours absorbed are not emitted leaving spaces in the spectrum as shown above. The diagram below shows an experiment done by Kirchhoff to find out the wavelengths of light absorbed by sodium. This experiment showed that the dark lines found in the absorption spectrum corresponded exactly to ...

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