= Brackett series
{c}
{title2=$n\to4$}
The <Brackett series> consists of hydrogen <spectral lines> with final <principal quantum number> four and initial <principal quantum number> greater than four. The <Rydberg formula> gives $1/\lambda=R_H(1/16-1/n^2)$. Its infrared <wavelengths> make these <recombination lines> less susceptible to <interstellar extinction> than optical <Balmer series> lines; quantitative use requires the appropriate <effective recombination coefficient>.
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