The Franck-Condon principle states that electronic transitions are so fast (10⁻¹⁵ s) that nuclear positions barely change — transitions are "vertical" on the potential energy diagram. The intensity of a vibronic line is proportional to the Franck-Condon factor |⟨ψ''_v''|ψ'_v'⟩|², the squared overlap of vibrational wavefunctions on the two electronic surfaces. Maximum overlap occurs not necessarily at v''=0 but where the upper-state wavefunction peaks over the lower-state classical turning point. The resulting vibrational progression (right panel) produces the characteristic envelope shape seen in UV-Vis spectra of diatomic molecules.