Normal Reflection Coefficient
The reflectivity of light from a surface depends upon the angle of incidence and upon the plane of polarization of the light. The general
expression for reflectivity is derivable from Fresnel's Equations. For
purposes such as the calculation of reflection losses from optical
instruments, it is usually sufficient to have the reflectivity at normal incidence. This normal incidence reflectivity is dependent upon the indices of refraction of the two media.
This calculation of the transmission is not exactly correct since it neglects the contribution from multiple internal reflections in the medium. Since this contribution is proportional to the square and higher powers of the reflection coefficient, it can often be neglected. The reflection loss at a single surface may seem small, but with multiple lenses, it becomes an unacceptable loss. Anti-reflection coatings are used to diminish the losses.