The Fermi's golden rule can be used for calculating the transition probability rate for an electron that is excited by a photon from the valence band to the conduction band in a direct band-gap semiconductor, and also for when the electron recombines with the hole and emits a photon. Consider a photon of frequency ω ω and wavevector q , where the light dispersion relation is ω ω = ( c / n ) | q | and n is the index of refraction.