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 hc = 1245 eV nm, e = 1.6 10-19 C).
122
First dip at 10.2 V electrons give 10.2 eV to excite H from n = 1 to n = 2
on returning (n = 2 1) it emits a photon of E = 10.2 eV
= hcE = 124510.2 nm 122 nm
Answer: 122
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