= Solution
A thin <antireflection coating> makes <reflections> from its two interfaces interfere destructively near a chosen <wavelength>, often in the green part of the visible spectrum. For a single lossless layer at <normal incidence>, $n_fd=\lambda_0/4$ and ideally $n_f=\sqrt{n_0n_s}$ minimize <reflection> at $\lambda_0$. <reflection> is less completely cancelled in the red and blue, so \b[the residual reflected light can appear purple]. This is primarily a <reflection> colour, not a claim that the transmitted beam is strongly purple. Multilayer coatings can broaden the low-reflectance band.
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