Friedrich Paschen investigated atomic spectra in strong magnetic fields. The Paschen-Back effect, associated with his work with Ernst Back, occurs when the external magnetic interaction competes with internal spin-orbit coupling. The weak-field description through total angular momentum then ceases to provide the simplest labels. Separate orbital angular momentum and spin projections become useful, changing the pattern of spectral lines. Comparing weak- and strong-field patterns tests the hierarchy of interactions within an atom.[ref]
A physicist.[ref]
https://uni-tuebingen.de/en/3260. Biographical identity and professional specialization
Portrait of Friedrich Paschen
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Friedrich Paschen (1865–1947) was a German physicist known for his contributions to spectroscopy and atomic physics. He is most famously associated with Paschen's Law, which describes the relationship between the pressure of a gas and the potential difference necessary to produce a spark across it. Paschen is particularly noted for identifying the Paschen series in the hydrogen spectrum, which concerns the wavelengths of light emitted by hydrogen when electrons transition between energy levels.