Sodium doublet at 589 nm

The 589.0 nm spectral line of the sodium yellow doublet is a transition from the 2P3/2
excited states to the 2S1/2 ground state of the sodium atom.
Energy level diagram showing how the two energy levels are split in the
presence of a weak magnetic field:

The 3P3/2 level is split because of L-S coupling.
The selection rules for this transition are:
Δ l = ±1
Δ j = ±1, 0

For the transition from 3P3/2 ===> 3S1/2,   Δ l = +1 (since S = 0 and P = 1)
and  Δ j = 0 for the 3P1/2 ===> 3S1/2    and +1 for the 3P3/2 ==> 3S1/2 transition. (j = s+l and is given by the subscript).
Therefore the transitions satisfy the selection rules.