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A single line in the <emission spectrum>.

So precise, so <discrete>, which makes no sense in <classical mechanics>!

Has been the leading motivation of the development of <quantum mechanics>, all the way from the:
* <Schrödinger equation>: major lines predicted, including <Zeeman effect>, but not finer line splits like <fine structure>
* <Dirac equation>: explains <fine structure> 2p spin split due to electron spin/orbit interactions, but not <Lamb shift>
* <quantum electrodynamics>: explains <Lamb shift>
* <hyperfine structure>: due to electron/nucleus spin interactions, offers a window into <nuclear spin>


Discussion on "Spectral line" by Ciro Santilli 32