Brackett series ($n\to 4$)

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.