Photon momentum ($\mathbf{p}=\hslash \mathbf{k}$)

In vacuum, a photon of wave vector $\mathbf{k}$ carries momentum $\mathbf{p}=\hslash \mathbf{k}$ and energy $E=\hslash c|\mathbf{k}|$. In a direct optical interband transition this momentum is generally small compared with relevant crystal momenta, motivating the nearly vertical-transition approximation in wave vector space. Electromagnetic momentum in matter requires additional conventions.