In the particular case of a two mirror cavity with equal reflectivity mirrors, the transmissivity is shown below for two different values of finesse. Finesse is a measure of how narrow the transmission line of the cavity is (as can be seen below). For the case of a two mirror cavity with equal reflectivity mirrors, it is given simply by $$F=\frac{4R}{1-R^2}=\frac{\delta\lambda}{\Delta\lambda},$$ where $$\delta\lambda$$ is the FWHM of the transmission peak and $$\Delta\lambda$$ is the free spectral range (as shown below). Since laser line mirrors can have reflectivities as high as 0.999995, it is possible to have cavities with a Finesse in the range of 100,000.
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