photon gas pressure and Independence on refraction index
Photons always travel at the speed of light, wherever they are. This is a consequence of them being massless particles.
Although &quot;light&quot; travels slower than $c$ in a refractive medium, the individual photons making up a light beam do not.
e.g. https://physics.stackexchange.com/questions/19203/what-is-the-mass-of-a-photon-in-non-empty-spaces