def prefactor(lamr, lama):
    """ Calculation prefactor for Mie potential: :math:`C_{Mie}=\lambda_r/(\lambda_r-\lambda_a) (\lambda_r/\lambda_a)^{\lambda_a/(\lambda_r-\lambda_a)}`
    """

    return lamr / (lamr - lama) * (lamr / lama)**(lama / (lamr - lama))