Editing Energy level (section)

==== Zeeman effect ====
{{Main|Zeeman effect}}
There is an interaction energy associated with the magnetic dipole moment, {{math|'''''μ'''''<sub>''L''</sub>}}, arising from the electronic orbital angular momentum, {{math|''L''}}, given by
: <math>U = -\boldsymbol{\mu}_L\cdot\mathbf{B}</math>
with
: <math>-\boldsymbol{\mu}_L = \dfrac{e\hbar}{2m}\mathbf{L} = \mu_B\mathbf{L}</math>.

Additionally taking into account the magnetic momentum arising from the electron spin.

Due to relativistic effects ([[Dirac equation]]), there is a magnetic momentum, {{math|'''''μ'''''<sub>''S''</sub>}}, arising from the electron spin
: <math>-\boldsymbol{\mu}_S = -\mu_\text{B} g_S \mathbf{S}</math>,
with {{math|''g''<sub>''S''</sub>}} the electron-spin [[g-factor (physics)|g-factor]] (about 2), resulting in a total magnetic moment, {{math|'''''μ'''''}},
: <math>\boldsymbol{\mu} = \boldsymbol{\mu}_L + \boldsymbol{\mu}_S</math>.

The interaction energy therefore becomes
: <math>U_B = -\boldsymbol{\mu}\cdot\mathbf{B} = \mu_\text{B} B (M_L + g_S M_S)</math>.