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# Hydrogen $\bigg( _1H^1\bigg)$, Deuterium $\bigg(_1H^2 \bigg)$, singly ionised Helium $\bigg(_2He^{4}\bigg)^+$ and doubly ionised . Consider an electron transition from $n = 2$ to $n = 1$. If the wave lengths of emitted radiation are $\lambda _1, \lambda_2, \lambda_3\;and \;\lambda_4$ respectively then approximately which one of the following is correct?

$\begin{array}{1 1}(A)\;\lambda_1=\lambda_2=4 \lambda_3=9 \lambda_4 \\ (B)\;\lambda_1=2 \lambda_2=3 \lambda_3=4 \lambda_4 \\(C)\;4 \lambda_1=2 \lambda_2=2 \lambda_3= \lambda_4 \\(D)\;\lambda_1=2 \lambda_2=2 \lambda_3 = \lambda_4 \end{array}$

$\lambda_1=\lambda_2=4 \lambda_3=9 \lambda_4$ is correct .
Hence A is the correct answer.