  Observations of interstellar extinction and polarization indicate that the
interstellar medium consists of aligned non-spherical dust grains which show
variation in the interstellar extinction curve for wavelengths ranging from NIR
to UV. To model the extinction and polarization, one cannot use the Mie theory
which assumes the grains as solid spheres. We have used a T-matrix based method
for computing the extinction efficiencies of spheroidal silicate and graphite
grains of different shapes (axial ratios) and sizes and used these efficiencies
to evaluate the interstellar extinction curve in the wavelength range 3.4--1.0
microns. A best fit linear combination of silicate and graphite grains of not
very large axial ratio, fits the observed extinction curve reasonably well.
  We calculate the volume extinction factor Vc, which is an important parameter
from the point of view of the cosmic abundance, for the spheroidal grain models
that reproduce the interstellar extinction curve. We find that the shape of the
grains do not affect the volume extinction factor.
  Finally we have also studied the extinction and linear polarization
efficiencies for aligned spheroids. The results show that the shape of grains
affects the linear polarization efficiencies considerably for various
orientation angles of the spheroids.

Astrophysics