= Solution
The approximate mirror relationship follows from the <Franck-Condon principle> when the ground and excited electronic states have similar vibrational frequencies and comparable potential shapes displaced along the active coordinates. Absorption samples $S_0,v\simeq0\to S_1,v'$ overlaps, while relaxed emission samples $S_1,v'\simeq0\to S_0,v$ overlaps. For displaced harmonic potentials, the corresponding overlap intensities form similar progressions on opposite sides of the transition between the lowest vibrational levels of the two electronic states.
\b[The symmetry is approximate and is most naturally compared on an energy or wavenumber axis.] A wavelength axis is nonlinear in photon energy, and solvent response, frequency changes, differing transition strengths and instrumental corrections can spoil an exact mirror image.
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