  There are indications that interstellar and interplanetary dust grains have
an inhomogeneous and fluffy structure. We investigate different methods to
describe light scattering by such composite particles. Both a model of layered
particles and discrete dipole calculations for particles with Rayleigh and
non-Rayleigh inclusions are used. The calculations demonstrate that porosity is
a key parameter for determining light scattering. We find that the optical
properties of the layered particles depend on the number and position of layers
if the number of layers is small ($\la 15$). For a larger number of layers the
scattering characteristics become independent of the layer sequence. The
optical properties of particles with inclusions depend on the size of
inclusions provided the porosity is large. The scattering characteristics of
very porous particles with inclusions of different sizes are found to be close
to those of multi-layered spheres. We compare the results of these calculations
with the predictions of the effective medium theories (EMT) which are often
used in astronomy as a tool to calculate the optical properties of composite
particles. The results of our analysis show that the internal structure of
grains (layers versus inclusions) only slightly affects the optics of particles
provided the porosity does not exceed 50%. It is also demonstrated that in this
case the optical properties of composite grains calculated with EMT agree with
the results of the exact method for layered particles. For larger porosity, the
standard EMT rules (i.e., Garnett and Bruggeman rules) give reliable results
for particles with Rayleigh inclusions only.
