#include <origin.h>
#include <math.h>

// Exact Fresnel Function
// Qz - current wave vector
// Qc - critical wave vector
// mu - absorption coefficient
// lambda - x-ray wave length

double Rf(double,double,double,double);
double Rf(double Qz,double Qc,double mu,double lambda)
{
double res,pp,pm,ac,ai,p,m;

if(lambda<=0.0)
	return(1.0);
if(Qz<0.0)
	return(1.0);

//Calculates critical angle
ac=asin(Qc/(4.0*pi/lambda));

//Calculates angle
ai=asin(Qz*lambda/(4.0*pi));

m=lambda*mu/(2.0*pi);
p=ai*ai-ac*ac;
pp=0.5*(sqrt(p*p+m*m/(4.0*pi*pi))+p);
pp=sqrt(pp);
pm=0.5*(sqrt(p*p+m*m/(4.0*pi*pi))-p);	
res=((ai-pp)*(ai-pp)+pm*pm)/((ai+pp)*(ai+pp)+pm*pm);

return(res);
}