def get_phase_refr_index(wlen):
    """Get the phase refractive index for ice as a function of wavelength.

    See: https://wiki.icecube.wisc.edu/index.php/Refractive_index_of_ice#Numerical_values_for_ice
    or eqn 3 in https://arxiv.org/abs/hep-ex/0008001v1

    Parameters
    ----------
    wlen
        Wavelength in units of nm

    Returns
    -------
    n
        Phase refractive index.

    """
    # Convert wavelength to micrometers
    wl_um = wlen/1000
    return 1.55749 - 1.57988*wl_um + 3.99993*wl_um**2 - 4.68271*wl_um**3 + 2.09354*wl_um**4