Confused on reflection coeffcient of Fresnel equations I am reading a paper about layered materials. However, I am confused on the implementation of this paper. The following picture describe the layered material. 

r_ab is a complex reflection coeffcient when going from medium a to medium b. 

And in the code, there is:

        float2 R12, phi12;
        fresnelDielectric(LdotH, 1.0, eta_2, R12, phi12);
        float2 R21 = R12;


I don't understand why is r_12 equal to r_21?
We have $t_{12} = t_{21}$, by the law of reciprocity. Also, $$r_{12} + t_{12} = 1\\r_{21} + t_{21} = 1$$
It follows that $r_{12} = r_{21}$ as well.