We study the extinction properties of the composite dust grains , consisting of host silicate spheroids and graphite as inclusions , using discrete dipole approximation ( DDA ) .
We calculate the extinction cross sections of the composite grains in the ultraviolet spectral region , 1200Å -3200Å and study the variation in extinction as a function of the volume fraction of the inclusions .
We compare the model extinction curves with the observed interstellar extinction curves obtained from the data given by the International Ultraviolet Explorer ( IUE ) satellite .
Our results for the composite grains show a distinct variation in the extinction efficiencies with the variation in the volume fraction of the inclusions .
In particular , it is found that the wavelength of peak absorption at ‘ 2175Å ’ shifts towards the longer wavelength with the variation in the volume fraction of inclusions .
We find that the composite grain models with the axial ratios viz .
1.33 and 2.0 fit the observed extinction reasonably well with a grain size distribution , a = 0.005-0.250 \mu m .
Moreover , our results of the composite grains clearly indicate that the inhomogeneity in the grain structure , composition and the surrounding media modifies the extinction properties of the grains .