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Effect of Dielectric Materials

It turns out that electromagnetic waves cannot propagate very far through a conducting medium before they are either absorbed or reflected. However, electromagnetic waves are able to propagate through transparent dielectric media without difficultly. The speed of electromagnetic waves propagating through a dielectric medium is given by
c' = \frac{c}{\sqrt{K}},
\end{displaymath} (330)

where $K$ is the dielectric constant of the medium in question, and $c$ the velocity of light in a vacuum. Since $K>1$ for dielectric materials, we conclude that:
The velocity with which electromagnetic waves propagate through a dielectric medium is always less than the velocity with which they propagate through a vacuum.

Richard Fitzpatrick 2007-07-14