Talk:Evanescent wave

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not clear from explanation why kz is larger than k when total internal reflection occurs.

It does gloss over that. Basically, if you solve the problem of matching the incident, reflected, and refracted waves at the boundaries, you find they all must have the form
\mathbf{E}(y=0) = \mathbf{E}_0 e^{- i k_z z}
But we also know from Snell's law that
n_1 \sin \theta_{incident} = n_2 \frac{k_{z}}{|\mathbf{k}_{refracted}|}
When that sinθincident on the left equals n2 / n1, k_z = |\mathbf{k}_{refracted}|. For n1 > n2, at larger incident angles, kz is bigger than |\mathbf{k}_{refracted}|.


[edit] Practical Applications

See this spectrum article.

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