Wavelength of the first line of Paschen series is $-\mathrm{\hspace{0.17em}\hspace{0.17em}}\left(\mathrm{R}=109700\mathrm{\hspace{0.17em}}{\mathrm{cm}}^{-1}\right)$

$$\begin{gathered} \overline{\mathrm{\nu }} =\frac{1}{\mathrm{\lambda }}=\mathrm{R}\left(\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right) \\ \mathrm{First} \mathrm{line} \mathrm{of} \mathrm{paschen} \mathrm{is} 4\left({\mathrm{n}}_2\right)\to 3 \left({\mathrm{n}}_1\right) \\ \overline{\mathrm{\nu }} =\frac{1}{\mathrm{\lambda }}=109700\left(\frac{1}{9}-\frac{1}{16}\right) \\ \mathrm{\lambda } =18752\times {10}^{-8}\mathrm{cm} \approx 18750{\mathrm{A}}^{\mathrm{o}} \end{gathered}$$