The first spectral line in the Pfund series of Hydrogen spectrum is given by
(R_H = Rydberg constant)

In Pfund series, the first spectral line forms when there is a transition from n=5 to n=6.

$$\begin{gathered} \frac{1}{\mathrm{\lambda }}={\mathrm{R}}_{\mathrm{H}}\left[\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right] \\ \frac{1}{\lambda }={\mathrm{R}}_{\mathrm{H}}\left[\frac{1}{5^2}-\frac{1}{6^2}\right]=\frac{11{\mathrm{R}}_{\mathrm{H}}}{900} \end{gathered}$$