Question

$E_{\text{emitted}} = \frac{50}{100} \times E_{\text{absorbed}}$

Or, $n_{1}.\frac{hc}{\lambda_{1}} = \frac{1}{2} \times n_{2} \times \frac{hc}{\lambda_{2}}$

Or, $\frac{\lambda_{1}}{\lambda_{2}} = \frac{2}{1} \times \frac{n_{1}}{n_{2}} = \frac{2}{1} \times \frac{2}{1} \Rightarrow \frac{\lambda_{1}}{x\overset{o}{A}} = \frac{4}{1} \Rightarrow \lambda_{1} = 4x\overset{o}{A}$