Vacuums have no matter or electric or magnetic fields. Media have subatomic-particle electric and magnetic fields {refractive index}| {index of refraction}, which attract and repel light-wave electric and magnetic fields, decreasing light speed. Refractive index depends on electrical permittivity and magnetic permeability. Vacuum has refractive index 1. Glasses have refractive index near 1.5. Dense polar salts have refractive index 2.5. Teflon is transparent to microwaves but has high refractive index. Plasmas and metals have negative permittivity. No natural substances have negative permeability.
speed
In materials, velocity v equals light speed in vacuum c divided by refractive index n: v = c/n.
Physical Sciences>Physics>Wave>Superposition>Refraction
5-Physics-Wave-Superposition-Refraction
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Date Modified: 2022.0224