def _F_N2(lbda_mu=0.3):
    """Depolarisation factor of N2 for Rayleigh scattering cross section
    
    Taken from `Bates 1984 <http://www.sciencedirect.com/science/article/pii/
    0032063384901028>`_
    
    Parameters
    ----------
    lbda_mu : float or ndarray
        wavelength in micrometers
    
    Returns
    -------
    float or ndarray
        depolarisation factor of N2  
    """
    return 1.034 + 0.000317 / lbda_mu ** 2
