The electron in the hydrogen atom jumps from excited state (n=3) to its ground state (n=1) and the photons thus emitted irradiate a photosensitive material. If the work function of the material is 5.1 eV, the stopping potential is estimated to be (the energy of the electron in nth state En = 13.6/n² eV)

Energy released when electron in the atom jumps from excited state (n=3) to ground state (n=1) is
E = hν = E3 - E1 = 13.6/3² - (13.6/1²) = 13.6(1/9 - 1) = 13.6(1-9)/9 = -13.6(8)/9 = -12.08 eV
The negative sign indicates that the energy is released. The magnitude of the energy is 12.08 eV.
Kinetic energy of emitted electrons = Energy of photon - work function
KE = 12.08 eV - 5.1 eV = 7 eV
Stopping potential (Vs) is given by:
KE = eVs
Therefore, Vs = KE/e = 7 eV / e = 7 V
Therefore, stopping potential is 7V