I was wondering why some websites are talking about the speed of light in teflon because it seems (tell me if I'm wrong) that it is an opaque material. Or are they talking about the light as an electromagnetic radiation in general and not particulary the visible light?
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$\begingroup$ Probably is refered as property of teflon that it's relative dielectric constant close to 1.0, therefore EM waves are not slowed down. $\endgroup$– Marko BuršičCommented Sep 23, 2015 at 18:02
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$\begingroup$ Yeah, they're essentially referring to the index of refraction at EM wavelengths which are not (significantly) absorbed by teflon. I recently dug up values of $n$ for colored plexiglass, in the GHz range, for example. $\endgroup$– Carl WitthoftCommented Sep 23, 2015 at 19:16
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$\begingroup$ Related: Can we measure speed of light in a non-transparent material $\endgroup$– BosoneandoCommented Sep 23, 2015 at 19:26
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The relative permittivity of Teflon is actually $\epsilon_r \approx 2.1$. The velocity of propagation at microwave & RF frequencies is going to be $v_p = \frac{c}{\sqrt\epsilon_r}$ or in terms of the index of refraction we have $v_p = \frac{c}{n}$ which is frequency dependent, and therefore $n_g$ which is the group velocity $n_g = n - \lambda_0\frac{dn}{d\lambda_0} \approx 1$ as the previous commentators suggested. Therefore it depends on the context in in which we're talking about Teflon, because it is frequently also used as a substrate for high frequency circuits because of its good loss characteristics ($\tan(\delta) = 0.00028$ at 3 GHz).
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The speed of light is the speed in which all electromagnetic waves travel, not only those in the visible light region of the frequency spectrum. Therefore, the speed of light in a material is an important characteristic for problems related to electromagnetism, regardless of your frequency range.
