c
(c) \(\frac{1}{\lambda } = R\,\left[ {\frac{1}{{n_1^2}} - \frac{1}{{n_2^2}}} \right]\) \(\frac{1}{\lambda } = 1.097 \times {10^7}{m^{ - 1}}\left[ {\frac{1}{{{1^2}}} - \frac{1}{{{\infty ^2}}}} \right]\) \(\therefore \) \(\lambda = 91 \times {10^{ - 9}}m\) We know \({10^{ - 9}} = 1\,nm\) So \(\lambda = 91\,nm\)