Abstract
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 (m(0) similar to 10(-6) m(e)). 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 m(0)c(2) energy is, E-K = m(0)c(2) (n-root n(2)-1/root n(2)-1). The skin depth inside the medium is delta(+) = (h root epsilon mu/m(0)) at very high frequency, and delta(+) = (root 2 h//m(0)c) at very low frequency. (C) 2016 Elsevier GmbH. All rights reserved.