![]() ![]() That being said, in most situations, the refractive index will be 1 or higher. And it's easy to see that this will always be less than 1. Where $N$ is the electron number density and $f$ is the EM frequency. Indeed, the formula for the refractive index of the ionosphere is given by the Appleton-Hartree equation, which for the ionosphere is The ionosphere of the earth has a refractive index of less than 1. ![]() \omega=2\pi f=\frac$, and so, to find a context in which the phase velocity will be greater than the speed of light, you need to find a context in which the refractive index is less than 1.Īnd we don't actually need to look very far to find an example of this.
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