One reasonable and memorable approximation excluding any <fine structure> is:
$$
E_n = -\frac{13.6eV}{n^2}
$$
{title=Hydrogen spectral series mnemonic}
see for example example: <hydrogen 1-2 spectral line>.


Gross hydrogen emission spectrum

{tag=Hydrogen emission spectrum}

= 21 centimeter line
{synonym}
{title2}

21 cm is very long and very low energy, because he energy split is very small!

Compare it e.g. with the <hydrogen 1-2 spectral line> which is 121.6 nm!


Ciro Santilli @cirosantilli 37

 Incoming links: Hydrogen 1-2 spectral line