sq(x) : x*x;
n = 1.52;			{ index of refraction }
rn = sq((1-n)/(1+n));		{ reflectivity at normal incidence }
tn(Tn) = (sqrt(sq(sq(1-rn))+4*sq(rn*Tn))-sq(1-rn)) / 2/rn/rn/Tn;