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# The angular momentum of an electron in a Bohr's orbit of H-atom is $4.2178\times 10^{-34}Kg-m^2/s$.Calculate the spectral line emitted when electron falls from this level to next lower level.

$\begin{array}{1 1}(a)\;1.8\times 10^{-2}cm&(b)\;1.8\times 10^{-4}cm\\(c)\;2.8\times 10^{-4}cm&(d)\;1.7\times 10^{-2}cm\end{array}$

$mur=\large\frac{nh}{2\pi}$
$\therefore \large\frac{nh}{2\pi}$$=4.2178\times 10^{-34} Or n=\large\frac{4.2178\times 10^{-34}\times 2\times 3.14}{6.625\times 10^{-34}} \Rightarrow 4 Thus \large\frac{1}{\lambda}=$$R_H\bigg[\large\frac{1}{n_1^2}-\frac{1}{n_2^2}\bigg]$
The transition spectral line for $4^{th}$ to $3^{rd}$ shell is
$\large\frac{1}{\lambda}$$=109678\big[\large\frac{1}{3^2}-\frac{1}{4^2}\big]$
$\Rightarrow \lambda=1.8\times 10^{-4}cm$