On the refractive index and photon mass

A new theoretical law for the refractive index is proposed. The law provides a limit to the photon mass inside optical materials. Experimental data reveal that the rest-mass energy of the photon inside a semiconductor or an insulator is found to be a few eV (m0 ∼ 10−6 me). This energy could be related to the band gap energy of the material. The electric (optical) conductivity of the material is found to be related to the photon mass. The photon kinetic energy inside the material, with a refractive index n, and rest-mass m0c2 energy is, EK=m0c2n−n2−1n2−1. The skin depth inside the medium is δ+=(ℏɛμ/m0) at very high frequency, and δ−=(2ℏ/m0c) at very low frequency.