In Franck-Hertz experiment, the first dip in the current-voltage graph for hydrogen is observed at 10.2 V. The wavelength of light emitted by…

In Franck-Hertz experiment, the first dip in the current-voltage graph for hydrogen is observed at $\displaystyle 10.2$ V. The wavelength of light emitted by hydrogen atom when excited to the first excitation level is $\displaystyle \_\_\_\_$ nm. (Given $\displaystyle \mathrm{hc}=1245 \mathrm{eV} \mathrm{nm}, \mathrm{e}=1.6 \times 10^{-19} \mathrm{C}$ ).