Ta có: \[\varepsilon = {E_n} - {E_m} \Rightarrow {E_n} = \varepsilon + {E_m} = 12,09 - 13,6 = - 1,51\,eV\]

Mà \[{E_n} = - \frac{{13,6}}{{{n^2}}} = - 1,51 \Rightarrow n \approx 3\]. Khi đó nguyên tử sẽ ở mức M

\[\varepsilon = \frac{\lambda } \Rightarrow {12,09.1,6.10^{ - 19}} = \frac{{{{6,625.10}^{ - 34}}{{.3.10}^8}}}{\lambda } \Rightarrow \lambda = {10^{ - 7}}\,m\]