= Sodium D-line Zeeman pattern
Neglect <hyperfine structure> and use a <magnetic field> weak compared with the <spin-orbit interaction>. The <sodium> D doublet has transitions $3p\,{}^2P_{1/2}\to3s\,{}^2S_{1/2}$ and $3p\,{}^2P_{3/2}\to3s\,{}^2S_{1/2}$. Their <Landé g-factors> are $2/3$, $4/3$ and $2$ respectively. With $m_u,m_l$ denoting upper and lower magnetic labels, the photon shift is
$$
\delta E_\gamma=\mu_BB(g_um_u-2m_l),\qquad m_u-m_l=0,\pm1.
$$
The first transition has four allowed shifts $\mu_BB\{\pm2/3,\pm4/3\}$; the second has six, $\mu_BB\{\pm1/3,\pm1,\pm5/3\}$. The most blue-shifted component of the second is $m_u=1/2\to m_l=-1/2$, with shift $5\mu_BB/3$. The apparently larger shift $m_u=3/2\to m_l=-1/2$ is excluded by the <electric-dipole selection rule>. The $3p\,{}^2P_{3/2}$ level lies above $3p\,{}^2P_{1/2}$, so in this weak-field model the $5/3$ component also has the shortest wavelength of the whole doublet. https://physics.nist.gov/PhysRefData/Handbook/Tables/sodiumtable5.htm[NIST, Energy Levels of Neutral Sodium, ordering of the two 3p fine-structure levels]{ref}
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