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We know ideally that if we stick a coating of thickness $d = \frac{\lambda}{4}$ with refractive index $n_{coat} = \sqrt{n_{air}n_{glass}} \approx 1.2$ there would be no reflected wave.

But it's hard to manufacture these coatings witn $n < 1.4$. Instead, porous anti-reflection coatings are used. How do they ensure low reflection?

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The explanation is not trivial, but here's one link to a relatively straightforward paper: http://www.chemistry.illinois.edu/research/materials/seminar_abstracts/documents/HeJabstract.pdf

The authors explain that the "porous" surface is a regular structure with features smaller than a wavelength, so that interferometric effects occur. You could think of it as a "pseudo-index of refraction". Compare, for example, a macroscopic antireflective device: the Fabry-Perot interferometer. In this case, the antireflectivity is strongly wavelength-dependent, but it does not depend on the bulk index of the materials in question.

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