Fabry-Perot Etalon

1. The problem statement, all variables and given/known data
A certain spectral line is known to consist of two equally intense components
with a wavenumber separation less than 20 m^-1: The Fabry-Perot fringes
produced by this line are photographed, using a plate separation of 25 mm. The
diameters of the smallest rings are found to be, in mm :
1:82; 3:30; 4:84; 5:57; 6:60; 7:15:
Explain why this experiment does not allow the wavenumber separation to be determined uniquely, but gives two possible values. What are they? Suggest a further experiment
which could be carried out to resolve the ambiguity.

2. Relevant equations
Wavenumber FSR=1/2nd

3. The attempt at a solution

So I think this has something to do with the free spectral range? If you work that out with the above equation, assuming the etalon is in air, you get 20m^01. So as the wavenumber separation is less than this, fringes from the two different wavenumbers will overlap. But then I don't know how to get the two possible answers.

Any help? Please?

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