def dustratio_to_dust1(dust2=0.0, dust_ratio=0.0, **extras):
    """Set the value of dust1 from the value of dust2 and dust_ratio

    Parameters
    ----------
    dust2 : float
        The diffuse dust V-band optical depth (the FSPS ``dust2`` parameter.)

    dust_ratio : float
        The ratio of the extra optical depth towards young stars to the diffuse
        optical depth affecting all stars.

    Returns
    -------
    dust1 : float
        The extra optical depth towards young stars (the FSPS ``dust1``
        parameter.)
    """
    return dust2 * dust_ratio