It is helpful but not strictly required to know:

$-mc^2\alpha^2/(2n^2) = -13.6\,\text{eV}/n^2$ $mc^2$ (b) 1. Delft (Hensen et al., Nature 526, 682, 2015) 1. Electron lines (solid lines with arrows): 11.1 14.1 14.2