def Knudsen(path, L):
    """Calculates Knudsen number or `Kn` for a fluid with mean free path
    `path` and for a characteristic length `L`.

    .. math::
        Kn = \\frac{\\lambda}{L}

    Parameters
    ----------
    path : float
        Mean free path between molecular collisions, [m]
    L : float
        Characteristic length, [m]

    Returns
    -------
    Kn : float
        Knudsen number []

    Notes
    -----
    Used in mass transfer calculations.

    .. math::
        Kn = \\frac{\\text{Mean free path length}}{\\text{Characteristic length}}

    Examples
    --------
    >>> Knudsen(1e-10, .001)
    1e-07

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
       Eighth Edition. McGraw-Hill Professional, 2007.
    .. [2] Cengel, Yunus, and John Cimbala. Fluid Mechanics: Fundamentals and
       Applications. Boston: McGraw Hill Higher Education, 2006.
    """
    return path / L
