Why does sodium light have two distinct wavelengths?

[kmp]

why sodium has two lines D1 and D2 ?

 

[ZapperZ]

The spin-orbit coupling splits the 3p orbital and removes the degeneracy. Read here:

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodium.html

Zz.

 

[kmp]

it may be due to transition between s1/2 and p3/2, p1/2 and p3/2 states

 

[guerom00]

Those are transitions from two fine levels.
That's $$^2P_{1/2} \to ^2S$$ and $$^2P_{3/2} \to ^2S$$

 

[Redbelly98]

In addition to ZapperZ's link, there is

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodzee.html#c1

which has this image: