(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A Fabry-Perot interferometer has spacing d = 2 cm between the glass plates, causing the direct and doubly reflected beams to interfere. As air is pumped out of the gap between the plates, the beams go through 23 cycles of constructive-destructive-constructive interference. If the wavelength of the light in the interfering beams is 5 × 10^−7 m, determine the index of refraction of the air initially in the interferometer.

2. Relevant equations

(I think) If constructive interference is occurring, 2d = mλ/n, for wave length λ, refractive index n and some integer m.

3. The attempt at a solution

I cannot figure out how to incorporate the following part in the equation: "As air is pumped out of the gap between the plates, the beams go through 23 cycles of constructive-destructive-constructive interference."

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# Fabry-Perot interferometer

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