In vacuum, the photon has a 4-momentum (E, p) with E^2 - p^2 = 0, i.e. it's massless. However, upon entering a medium of refractive index n, we expect that the photon retains its energy, while reducing its momentum by a factor n (due to increased wavelength). We then have for the 4-momentum of a photon in the medium (E, p/n) with E^2 - p^2 / n^2 != 0 which implies that the photon gains mass in a medium. Would it be correct to identify this particle not as a "photon in a medium" but as a polariton? PS I'm a particle physicist, so take it easy on me :) Thanks!