  Optical properties of graphene are explored by using the generalized
tight-binding model. The main features of spectral structures, the form,
frequency, number and intensity, are greatly enriched by the complex
relationship among the interlayer atomic interactions, the magnetic
quantization and the Coulomb potential energy. Absorption spectra have the
shoulders, asymmetric peaks and logarithmic peaks, coming from the band-edge
states of parabolic dispersions, the constant-energy loops and the saddle
points, respectively. The initial forbidden excitation region is only revealed
in even-layer AA stacking systems. Optical gaps and special structures can be
generated by an electric field. The delta-function-like structures in
magneto-optical spectra, which present the single, twin and double peaks, are
associated with the symmetric, asymmetric and splitting Landau-level energy
spectra, respectively. The single peaks due to the non-tilted Dirac cones
exhibit the nearly uniform intensity. The AAB stacking possesses more
absorption structures, compared to the other stackings. The diverse
magneto-optical selection rules are mainly determined by the well-behaved,
perturbed and undefined Landau modes. The frequent anti-crossings in the
magnetic- and electric-field-dependent energy spectra lead to the increase of
absorption peaks and the reduced intensities. Part of theoretical calculations
are consistent with the experimental measurements, and the others need further
detailed examinations.
