def refr_idx_300ppm_co2(lbda_mu=0.3):
    """Get refractive index of air using eq. of Peck and Reeder, 1972
    
    Uses either of two parametrisations for wavelengths above or below 0.23 
    microns using parametrisations from `Peck and Reeder, 1972, Dispersion of 
    air <https://www.osapublishing.org/josa/abstract.cfm?uri=josa-62-8-958>`_.
    
    Parameters 
    ----------
    lbda_mu : float or ndarray
        wavelength in microns
    
    Returns
    -------
    float
        refractive index
    """
    if lbda_mu >= 0.23:
        return 1 + (5791817 / (238.0185 - (1 / lbda_mu) ** 2) + 167909 / (
            57.362 - (1 / lbda_mu) ** 2)) * 10 ** -8
    else:
        return 1 + (8060.51 + 2480990 / (132.274 - (1 / lbda_mu) ** 2) + 
            17455.7 / (39.32957 - (1 / lbda_mu) ** 2)) * 10 ** -8
