Diagram of a Fabry-Perot interferometer showing two transmitted beams (T0 and T1) the second of which has been internally reflected twice. Also shown is the effective distance (l0) by which the amplitude of the second beam at point c must be retarded when added to the first.

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Diagram of a Fabry-Perot interferometer showing two transmitted beams (T0 and T1) the second of which has been internally reflected twice. Also shown is the effective distance (l0) by which the amplitude of the second beam at point c must be retarded when added to the first.