def cauchy(wvl, A, *args):
    """Cauchy's equation for the (real) index of refraction of transparent materials.

    Parameters
    ----------
    wvl : `number`
        wavelength of light, microns
    A : `number`
        the first term in Cauchy's equation
    args : `number`
        B, C, ... terms in Cauchy's equation

    Returns
    -------
    `numpy.ndarray`
        array of refractive indices of the same shape as wvl

    """
    seed = A

    for idx, arg in enumerate(args):
        # compute the power from the index, want to map:
        # 0 -> 2
        # 1 -> 4
        # 2 -> 6
        # ...
        power = 2*idx + 2
        seed = seed + arg / wvl ** power

    return seed