  We study the extinction efficiencies as well as scattering properties of
particles of different porosity. Calculations are performed for porous
pseudospheres with small size (Rayleigh) inclusions using the discrete dipole
approximation. Five refractive indices of materials covering the range from
$1.20+0.00i$ to $1.75+0.58i$ were selected. They correspond to biological
particles, dirty ice, silicate, amorphous carbon and soot in the visual part of
spectrum. We attempt to describe the optical properties of such particles using
Lorenz-Mie theory and a refractive index found from some effective medium
theory (EMT) assuming the particle is homogeneous. We refer to this as the
effective model.
  It is found that the deviations are minimal when utilizing the EMT based on
the Bruggeman mixing rule. Usually the deviations in extinction factor do not
exceed $\sim 5%$ for particle porosity ${\cal P}=0 - 0.9$ and size parameters
$x_{\rm porous} = 2 \pi r_{\rm s, porous}/\lambda \la 25$. The deviations are
larger for scattering and absorption efficiencies and smaller for particle
albedo and asymmetry parameter. Our calculations made for spheroids confirm
these conclusions. Preliminary consideration shows that the effective model
represents the intensity and polarization of radiation scattered by fluffy
aggregates quite well. Thus, the effective models of spherical and
non-spherical particles can be used to significantly simplify computations of
the optical properties of aggregates containing only Rayleigh inclusions.
