def sellmeier(lmbda, B1, B2, B3, C1, C2, C3):
    """
    Estimate refractive index of glass with wavelength.

    Using the Sellmeier equation.

    Input:
    lmbda: the Wavelength
    B(1-3) : B coefficients of glass
    C(1-3) : C coefficients of glass

    Returns:
    n : Fitted refractive index
    """
    n_squ = B1*lmbda**2/(lmbda**2-C1) + B2*lmbda**2/(lmbda**2-C2) + B3*lmbda**2/(lmbda**2-C3) + 1

    return(n_squ**0.5)