function [Mt, Ms] = transmission(R, a1, n1, n2)
% [Mt, Ms] = transmission(R, a1, n1, n2)

if length(R) > 1
    Rt = R(1);
    Rs = R(2);
else
    Rt = R;
    Rs = R;
end

% degrees to radians
a1 = a1 * pi / 180;

% output angle from refraction (snell's law)
a2 = asin(n1*sin(a1)/n2);

% define angle/index change
dn = (n2*cos(a2) - n1*cos(a1)) / (cos(a1)*cos(a2));

% effective RoC
Ret = Rt/dn;
Res = Rs/dn;

% tangential
Mt = [cos(a2)/cos(a1) 0; 1/Ret cos(a1)/cos(a2)];
% sagittal
Ms = [1 0; 1/Res 1];