Given : $\frac{r_i}{r_f}=\frac{16}{4}=\frac{n_1^2}{n_f^2}$ $$ \begin{aligned} & \frac{\mathrm{n}_{\mathrm{i}}}{\mathrm{n}_{\mathrm{f}}}=\frac{4}{2} \\ & \frac{1}{\lambda}=\mathrm{R}\left(\frac{1}{\mathrm{n}_{\mathrm{f}}^2}-\frac{1}{\mathrm{n}_{\mathrm{i}}^2}\right)=1.0973 \times 10^7\left(\frac{1}{(2)^2}-\frac{1}{(4)^2}\right) \\ & \lambda=4.86 \times 10^{-7} \mathrm{~m} \approx 486 \mathrm{~nm} \end{aligned} $$