Heat treatment of muscular pain involves radiation of wavelength of about 900 nm. Which spectral line of H-atom is suitable for this purpose
[R_H = 1x 10⁵cm^-1,h = 6.6x10^-34 Js, c = 3 x 10⁸ms^-1 ]

The wavelength of visible range is 360-720 nm, so wavelength of given radiation is in IR region. So, the line must not belong to Lyman and Balmer series.
${\displaystyle \frac{1}{\lambda }=R_H[\frac{1}{9}-\frac{1}{{\infty }^2}]=\frac{R_H}{9}}$
${\displaystyle \lambda =\frac{9}{R_H}=\frac{9}{{10}^{5}cm}=9\times {10}^{-7}}$m =900 nm
So, given spectral line of H-atom is ∞→3${\displaystyle \infty \to 3}$ , Paschen series