Drawing the interference pattern of light falling on a grid

// Example drawing the interference pattern of light
// falling on a grid with n slits and ratio r of slit
// width over distance between slits.
// function code in C

double single(double *x, double *par)
{
double const pi=4*atan(1.);
return pow(sin(pi*par[0]*x[0])/(pi*par[0]*x[0]),2);
}

double nslit0(double *x,double *par)
{
double const pi=4*atan(1.);
return pow(sin(pi*par[1]*x[0])/sin(pi*x[0]),2);
}

double nslit(double *x, double *par)
{
return single(x,par) * nslit0(x,par);
}

// This is the main program

void slits()
{
float r,ns;

// request user input
cout << "slit width / g ? ";
scanf("%f",&r);
cout << "# of slits ? ";
scanf("%f",&ns);
cout <<"interference pattern for "<< ns
<<" slits, width/distance: "<<r<<endl;

// define function and set options
TF1 *Fnslit = new TF1("Fnslit",nslit,-5.001,5.,2);
Fnslit->SetNpx(500);

// set parameters, as read in above
Fnslit->SetParameter(0,r);
Fnslit->SetParameter(1,ns);

// draw the interference pattern for a grid with n slits
Fnslit->Draw();

}