def N2_depolarisation(wavelength):
    """
    Returns the depolarisation of nitrogen :math:`N_2` as function of
    wavelength :math:`\lambda` in micrometers (:math:`\mu m`).

    Parameters
    ----------
    wavelength : numeric
        Wavelength :math:`\lambda` in micrometers (:math:`\mu m`).

    Returns
    -------
    numeric
        Nitrogen :math:`N_2` depolarisation.

    Examples
    --------
    >>> N2_depolarisation(0.555)  # doctest: +ELLIPSIS
    1.0350291...
    """

    wl = wavelength
    N2 = 1.034 + 3.17 * 1.0e-4 * (1 / wl ** 2)

    return N2