I understand that the speed of light in a medium like glass, $c_n$, is reduced by the refractive index $n$ so that we have: $$c_n=\frac{c}{n},$$ where $c$ is the speed of light in the vacuum.
Is it true that in the medium $$E_n = |\vec{p_n}|\ c_n,$$ where $E_n$ is the energy of a photon in the medium and $\vec{p_n}$ is its momentum?
