A coefficient describing the reflectivity of the mirrors in a Fabry-Perot interferometer and equal to
where r is a Fresnel reflection coefficient from the Fresnel equations, so that the reflection and transmission ratios may be simply expressed as
with d the thickness of the glass the light is transmitted through, n is its index of refraction, and is the wavelength of the light.
Equation (3) defines the Airy function
Fabry-Perot Interferometer, Multiple Reflection Interferometer
References
Born, M. and Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference, and Diffraction of Light, 7th ed. Cambridge, England: Cambridge University Press, 1999.
