def vpvs2poisson(vpvs_ratio):
    """
    Convert Vp/Vs ratio to Poisson's ratio.

    Parameters
    ----------
    vpvs_ratio : float
        Vp/Vs ratio.

    Returns
    -------
    poisson_ratio : float
        Poisson's ratio.
    """
    s = vpvs_ratio ** 2
    return 0.5 * (s - 2) / (s - 1)