Index δ′ of the power law describing the effective attenuation curve versus the attenuation in the V band for the Charlot & Fall (2000) model. The size of the points is proportional to the V band ISM attenuation from 0.01 mag to 2 mag and the color gives μ, the fraction of the dust in the ISM following the color bar to the right of the figure. For reference, a starburst curve corresponds to δ′≃ − 0.7. We see that in the low A_V and low μ regime, the attenuation curve can become much steeper.