Q30. 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 hydrogen atom when excited to the first excitation level is $\_\_\_\_$ nm .\\
(Given $\mathrm { hc } = 1245 \mathrm { eVnm } , \mathrm { e } = 1.6 \times 10 ^ { - 19 } \mathrm { C }$ ).\\