function n=n_sf6(lambda)

C1 = 1.72448482;
C2 = 0.0134871947;
C3 = 0.390104889;
C4 = 0.0569318095;
C5 = 1.04572858;
C6 = 118.557185;

n=sqrt(1+C1*lambda.^2./(lambda.^2-C2) + C3*lambda.^2./(lambda.^2-C4) + ...
    C5*lambda.^2./(lambda.^2-C6));