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So here's the question:-

A thin film of transparent material of refractive index $1.52$ and thickness $0.42\ \rm \mu m$ forms a thin coating on glass of refractive index $1.60$. It is viewed by reflection with white light at normal incidence. What visible wavelength in vacuum is most strongly reflected?

Please could someone explain the meaning (I am not asking for an answer) as this question seems quite ambiguous.

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    $\begingroup$ The question is testing your understanding of the interference of light reflected at the two interfaces. $\endgroup$ – my2cts Sep 30 '18 at 8:30
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This question is about thin film reflection - which wavelengths would reflect better at certain incidences for certain film thickness, which is normally in the same range as the wavelengths.

https://en.wikipedia.org/wiki/Thin-film_interference

Is the wikipedia article for this one and related things.

It requires you to calculate the new wavelength in the film, realize that light reflects both on top of the film and after a reflection under the film, and then figure out at which wavelengths you have a harmonic. You'll see far stronger reflection for the harmonics.

This effect is often used in highly reflector mirrors - for example, lasers classically have an optical resonator, which is then created by two mirrors that use multiple thin layers to create a harmonic for the wavelength of the laser.

More specifically, taking this image from the above-mentioned wikipedia page: Thin film interference

Your angle is $0$, your $d$ is $4.2\times 10^{-7}$, your $n_1 = 1$ and your $n_2 = 1.52$.

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