7.11

$${\begin{array}{l} hf &=& 2,17 \cdot 10^{-18} \left (\frac {1} {n^2_1} - \frac {1} {n^2_1} \right) J &=& 13,6 \cdot \left (\frac {1} {n^2_1} - \frac {1} {n^2_1} \right) eV \end{array} }$$