def fresnel_criterion(W, z, wav):
    """
    determine the frensel number of a wavefield propagated over some distance z

    :param W: approx. beam/aperture size [m]
    :param z: propagation distance [m]
    :param wav: source wavelength [m]

    :returns F: Fresnel number
    """

    F = W**2/(z*wav)
    return F