where is the index of refraction for a layer with total scattering length density .
The Fresnel reflectiviy between any two layers is given by
The total reflectivity is calculated from back to front assuming there is no refelctivity from the backmost () layer () since it is infinite in extent.
The reflectivity of the entire system is given by the reflectivity from the top surface, and is shown in the last line above. Here, and are real valued, and everything else, including , is complex.