Interference of a wave train of finite length in a fabryperot interferometer

The spectral compositon of radiation is correlated with the length of a train of waves for evaluating the relative widening and decrease in visibility of the pass band of a Fabry - Perot interferometer. The degree of monochromaticity of the light is expressed through the number of coherent waves, which is determined by the ratio of the coherence length to the path difference in the interferometer. Inasmuch as this number tends to infinity for null path differences, the degree of monochromaticity is relative. We derive analytic formulas which describe the widening of the bands with increasing path difference. We give a relation between the half - widths of the apparatus, spectral, and observable functions. We examine the workings of interference mirrors and filters of light for the case of quasimonochromatic radiation.