# Spectral lines in Franck-Hertz expreiment

In a Franck-Hertz experiment in which mercury vapor has been replaced by atomic hydrogen, it is observed pronounced maximum in the current that circulates through the galvanometer for potential values of 10.2 V and 12.09 V; and if it analyzes spectroscopically the emergent radiation three spectral lines are observed. Explain the results.

I'm not sure about the three spectral lines if there are only two potential values; i.e for me there are only two wavelengths using the equation $E= h\frac{c}{\lambda}$. And the energy is obtained by multiplying the potential value with the charge of the electron $E= eV$.