Question

Heat treatment of muscular pain involves radiation of wavelength of about $900\, nm$. Which spectral line of H-atom is suitable for this purpose ? $[R_H = 1 \times 10^5 \; cm^{-1}, h = 6.6 \times 10^{-34} \; Js, c = 3 \times 10^8 \; ms^{-1}]$

• A. Paschen, $5\to 3$
• B. Paschen, $\infty \to 3$
• C. Lyman, $\infty \to 1$
• D. Balmer, $\infty \to 2$

Answer 1

Answer: (B)

Paschen, $\infty \to 3$

Answer 2

We have,
$\frac{1}{\lambda}=R Z^{2}\left(\frac{1}{n_{l}{ }^{2}}-\frac{1}{n_{f}{ }^{2}}\right)$
Here if $n _{l}=3$ and $n _{ f }=\infty$
$\frac{1}{\lambda}=10^{-7} \times 1^{2}\left(\frac{1^{2}}{3}-\frac{1}{\infty^{2}}\right)=\frac{10^{-7}}{9}=900\, mm$