The expression for energy of photon when switch S1 is closed is, E=hv0+(5+1)eV......(I) The expression for energy of photon when switch S2 is closed and S1 is open is, 2E=hv0+(20−5)eV......(II) On solving equation (I) and (II), we get hv0=3eV v0=
3eV
h
v0=
3×1.6×10−16V
6.626×10−34J⋅s
v0=7.25×1014Hz The wavelength is calculated as, λ=