Hydrogen (_1 H^1), Deuterium (_1 H^2), singly ionised Helium (_2 He^4)^+ and doubly ionised lithium (_3 Li^6)^++ all have one electron around the…

Hydrogen $\displaystyle \left({ }_1 \mathrm{H}^1\right)$, Deuterium $\displaystyle \left({ }_1 \mathrm{H}^2\right)$, singly ionised Helium $\displaystyle \left({ }_2 \mathrm{He}^4\right)^{+}$and doubly ionised lithium $\displaystyle \left({ }_3 \mathrm{Li}^6\right)^{++}$all have one electron around the nucleus. Consider an electron transition from $\displaystyle \mathrm{n}=2$ to $\displaystyle \mathrm{n}=1$. If the wave lengths of emitted radiation are $\displaystyle \lambda_1, \lambda_2, \lambda_3$ and $\displaystyle \lambda_4$ respectively then approximately which one of the following is correct ?