[Physics] How to only one valence electron in sodium cause doublet in spectrum

atomic-physicsatomsspectroscopy

The valence electron in sodium atom gets excited and moves to higher orbital say $3P$ and it then comes to the lower energy state $3S$ thus there should be only one line in spectrum (regarding this particular transition). But my book says since $3P$ can exist in 2 states $3P_{1/2}$ and $3P_{3/2}$ so there are doublet in the spectrum but how can only one electron make two transitions – from $3P_{1/2}$ to $3S$ and from $3P_{3/2}$ to $3S$ ?

Best Answer

It is not one electron making the transitions. For each atom electrons can exist in two P states and if a transition to an S state is possible the one of quantum jumps occurs. So when you see the spectrum doublet it is the result of these quantum jumps by electrons in different atoms.