  We study theoretically a fundamental issue in solids: the evolution of the
optical spectra of polaron as the electron-phonon coupling increases. By
comparing the exact results obtained by diagrammatic Monte Carlo method and the
data got through exact diagonalization within an appropriate subspace of the
phononic wavefunctions, the physical nature of the crossover from weak to
strong coupling is revealed. The optical spectra are well understood by the
quantum mechanical superposition of states with light and heavy phonon clouds
corresponding to large and small polarons, respectively. It is also found that
the strong coupling Franck-Condon regime is not accessible at realistic values
of the coupling constant.
