Q_ext (thick solid lines), Q_sca (dotted lines), Q_abs (dashed lines) and Q_sca/Q_ext (thin solid lines) vs. λ^-1 for 30 nm radius particles as a function of band gap.The vertical grey line shows the central position of the UV bump at 217 nm and the vertical, lighter grey bands on either side indicate the full width of the observed UV bump (Fitzpatrick 2004; Fitzpatrick & Massa 2007). The thin horizontal black line shows the limit where Q_sca/Q_ext = 1.