Lyman and Balmer series Why is it that the Lyman series for hydrogen atom is available for both absorption and emission, but the Balmer series is only available for emission? 
The book I'm reading doesn't contain any detail about such a discrepancy. Our professor didn't teach this either. I looked  this up on the internet but I did not come across any valid answer. 
Any help would be appreciated.
 A: The Lyman series in either absorption or emission, is defined by having the lower of the two levels involved with quantum number $n_L = 1$, the lowest or ground state.
On the other hand, the Balmer series in either absorption or emission is defined by having the lower of the two levels involved with quantum number $n_L = 2$, the first excited state.
So, for emission, if a hydrogen atom is excited to some high quantum number, the electron will drop back down through the various levels to eventually reach the ground state, $n_L = 1$
Any jump from some excited state to the $n_L = 1$ state will cause the emission of a line in the Lyman series;  any jump from some excited state to the $n_L = 2$ state will cause the emission of a line in the Balmer series.
The reverse is true for absorption. The atom starts in either the $n_L=1$ or $n_L=2$ level and jumps to some higher level as it absorbs a particular wavelength. However, there are no hydrogen atoms in the $n_L = 2$ level to absorb the Balmer series wavelengths.  Any such atom would almost instantly drop down into the ground state.
