  Matrix elements of electron-light interactions for armchair and zigzag
graphene nanoribbons are constructed analytically using a tight-binding model.
The changes in wavenumber ($\Delta n$) and pseudospin are the necessary
elements if we are to understand the optical selection rule. It is shown that
an incident light with a specific polarization and energy, induces an indirect
transition ($\Delta n=\pm1$), which results in a characteristic peak in
absorption spectra. Such a peak provides evidence that the electron standing
wave is formed by multiple reflections at both edges of a ribbon. It is also
suggested that the absorption of low-energy light is sensitive to the position
of the Fermi energy, direction of light polarization, and irregularities in the
edge. The effect of depolarization on the absorption peak is briefly discussed.
