Physics - Modern Physics Question with Solution | TestHub

The photons' energies correspond to transitions in the hydrogen atom. 12.1 eV is the energy difference between $n=3$ and $n=1$. 10.2 eV is the energy difference between $n=2$ and $n=1$. 1.9 eV is the energy difference between $n=3$ and $n=2$.

The 12.1 eV and 10.2 eV photons could come from two different atoms. Alternatively, all three photons could come from a single atom undergoing transitions $3\to 1$ (12.1 eV), or $3\to 2$ (1.9 eV) followed by $2\to 1$ (10.2 eV).