We noted in Section 18.104.22.168
that ideal diffuse reflection at textured surfaces of an absorber material can trap light and thereby enhance absorption. An analysis by Yablonovitch using geometrical (ray) optics showed this enhancement can be as much as 4n2
is the absorber’s index of refraction 
. Since geometrical optics is used, it is inherently assumed that the thickness of the absorber t
is greater than the wavelength λ
of the radiation, which may not be true in some light-trapping structures. In this appendix, we derive this 4n2
factor following the approach of Ref. 
. In so doing, we will take the opportunity to explore the interpretations, implications, and ...