The electron in the $\mathrm{n}^{\text {th }}$ orbit of $\mathrm{Li}^{2+}$ is excited to ( $\mathrm{n}+1$ ) orbit using the radiation of energy $1.47 \times 10^{-17} \mathrm{~J}$ (as shown in the diagram). The value of $n$ is $\_\_\_\_$ .
Given $R_H=2.18 \times 10^{-18} \mathrm{~J}$
