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Observations and theoretical principles indicate that electromagnetic waves, including light, propagate in dielectric media at speeds lower than those in free space, as eloquently expressed in Maxwell's equations featuring material-dependent permittivity and permeability. This study reveals that the observed slower propagation of waves in dielectric media arises from an interference between two types of waves: a forward-moving primary wave and a set of secondary waves induced by the primary wave's interaction with the medium. The forward-moving primary wave and secondary waves travel at the speed of light in vacuum. Notably, the reflected wave caused by the impedance discontinuity on the boundary of a dielectric medium is manifested by the induced secondary waves moving in the opposite direction to the primary wave. From a photonic point of view, photons will be moving back and forth below the slowly moving apparent front of the observed wave with the speed of light in vacuum. Ahead of this front the probability of finding photons is zero due to complete destructive interference of the waves in that region. From a photonic perspective, beneath the gradually advancing facade of the observed wave, photons move at the unyielding speed of light in vacuum. The likelihood of detecting photons ahead of the front diminishes to zero, a consequence of the thorough destructive interference manifesting within that specific region.
Cooray et al. (Tue,) studied this question.