Photon momentum
= Photon momentum
{title2=$\mathbf p=\hbar\mathbf k$}
In <vacuum>, a <photon> of <wave vector> $\mathbf k$ carries <momentum> $\mathbf p=\hbar\mathbf k$ and <energy> $E=\hbar c|\mathbf k|$. 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.