Optical depth as function of wavelength for three representative dips. The observed values are shown by grey dots with error bars. The red dashed line shows the extinction opacity (absorption + scattering) for the grain size distribution that best fit the data in the wavelength interval 400900 nm (limits shown by the vertical grey dotted lines), normalised to the observed value at 700 nm. The values of a_min, a_max, q are given on top of each panel; the corresponding grain size distributions (n(a)) are displayed in the inset plots.