Does light slowdown in dielectric media?

Abstract

Observations and theoretical principles suggest that electromagnetic waves, including light, travel more slowly in dielectric media than in vacuum. Maxwell's equations, incorporating material dependent permittivity and permeability, elegantly capture this effect. Previous studies indicate that the observed slower speed is due to interference effects, with the actual electromagnetic fields in the dielectric propagating at the speed of light in vacuum. However, these studies did not provide explicit expressions for the field components moving at this speed. The aim of the present study is to address this gap by analysing the structure of the electromagnetic field components within the dielectric. We examine how these components, each traveling at the speed of light in vacuum, interact to produce a net field that appears to propagate more slowly. Our findings show that the observed slower propagation in dielectric media results from interference between two types of waves: a forward moving incident wave and a set of secondary waves, moving both in the forward and backward directions, and induced by the interaction of the incident wave with the dielectric medium. Both the incident wave and the secondary waves travel at the speed of light in vacuum. Importantly, we observe that the reflected wave caused by the impedance discontinuity at the boundary of the dielectric medium arises from secondary waves moving in the direction opposite to the incident wave.