def powerlaw(wave, tau_v=1, alpha=1.0, **kwargs):
    """Simple power-law attenuation, normalized to 5500\AA.

    :param wave:
        The wavelengths at which optical depth estimates are desired.

    :param tau_v: (default: 1)
        The optical depth at 5500\AA, used to normalize the
        attenuation curve.

    :returns tau:
        The optical depth at each wavelength.
    """
    return tau_v * (wave / 5500)**(-alpha)