Fabry-Perot Interferometer
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When light of wavelength is incident on the parallel plates of Fabry-Perot interferometer, it
undergoes multiple reflections by the semi reflecting surfaces. Light will only be transmitted
across to other side when all the multiply transmitted waves arrive together in phase.
$$\Delta =d \cos \cos \Theta \\
2d\cos \cos \Theta = Nλ $$
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Circular fringes can be observed on a screen placed at a distance \(D\) from the interferometer. The
radius of the first circular fringe is taken as \(R_m\) and rest as \(R_{m+n}\).
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The radius (\(R_n\)) of the \(n^{th}\) bright fringe (also called fringe of order \(n\)), distance between the
etalon and the screen (\(D\)) and the thickness of etalon (\(d\)) are related by
$$d= \frac{nD^2\lambda }{R_{m+n}^2-R_{n}^2}$$