text: The Lyman-alpha transition in hydrogen in the presence of the spin–orbit interaction involves the transitions
  : and 
  In the presence of an external magnetic field, the weak-field Zeeman effect splits the 1S and 2P levels into 2 states each () and the 2P level into 4 states ().  The Landé g-factors for the three levels are:
  : for  (j=1/2, l=0)
  : for  (j=1/2, l=1)
  : for  (j=3/2, l=1).
  Note in particular that the size of the energy splitting is different for the different orbitals, because the g values are different. On the left, fine structure splitting is depicted. This splitting occurs even in the absence of a magnetic field, as it is due to spin–orbit coupling. Depicted on the right is the additional Zeeman splitting, which occurs in the presence of magnetic fields.