Every series of hydrogen spectrum has an upper and lower limit in wavelength. The spectral series which has an upper limit of wavelength equal to 18752 Å is:  (Rydberg constant R = 1.097 × 10⁷ per metre)

Paschen series

$\frac{1}{\lambda}=R\left[\frac{1}{n_1^2}-\frac{1}{n_2^2}\right] \Rightarrow \frac{1}{n_1^2}-\frac{1}{n_2^2}=\frac{1}{R \lambda}$

$=\frac{1}{1.097 \times 10^7 \times 18752 \times 10^{-10}}=0.0486=\frac{7}{144}$. But $\frac{1}{3^2}-\frac{1}{4^2}=\frac{7}{144} \Rightarrow n_1=3 \text { and } n_2=4 \text { (Paschen series) }$