function [Mt, Ms] = mirror(R, a1, n)
% [Mt, Ms] = mirror(R, a1, n)

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

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

% tangential
Mt = [1 0; -2*n/cos(a1)/Rt 1];
% sagittal
Ms = [1 0; -2*n*cos(a1)/Rs 1];