In case of a direct band gap semiconductor, the light of energy more than the bandgap can be directly absorbed to transfer a valence band electron to the conduction band resulting in the generation of an electron–hole pair. While in indirect-band-gap semiconductors, where the valence and conduction band extrema are separated in k-space, the participation of both the photons and the phonons is necessary for such transitions to take place. In order to get a clear understanding of the process of exciton generation through photon absorption, one needs to understand how light interact with the band electrons.