Physics - Modern Physics Question with Solution | TestHub

6 lines $\Rightarrow {}^4{C}_2$, so the highest level reached is $n=4$ (B). Had the atoms started in $n=1$, no emitted line could be shorter than ${\lambda }_0$; starting from $n=2$, the transitions ending below $n=2(4\to 1,3\to 1)$ give exactly two wavelengths shorter than ${\lambda }_0$, so the initial state is $n=2(\mathrm{C})$.