  In protoplanetary disks micron-size dust grains coagulate to form larger
structures with complex shapes and compositions. The coagulation process
changes the absorption and scattering properties of particles in the disk in
significant ways. To properly interpret observations of protoplanetary disks
and to place these observations in the context of the first steps of planet
formation, it is crucial to understand the optical properties of these complex
structures. We derive the optical properties of dust aggregates using detailed
computations of aggregate structures and compare these computa- tionally
demanding results with approximate methods that are cheaper to compute in
practice. In this way we wish to understand the merits and problems of
approximate methods and define the context in which they can or cannot be used
to analyze observations of objects where significant grain growth is taking
place. For the detailed computations we used the discrete dipole approximation
(DDA), a method able to compute the interaction of light with a complexly
shaped, inhomogeneous particle. We compared the results to those obtained using
spherical and irregular, homogeneous and inhomogeneous particles. While no
approximate method properly reproduces all characteristics of large dust
aggregates, the thermal properties of dust can be analyzed using irregularly
shaped, porous, inhomogeneous grains. The asymmetry of the scattering phase
function is a good indicator of aggregate size, while the degree of
polarization is probably determined by the size of the constituent particles.
Optical properties derived from aggregates significantly differ from the most
frequently used standard ("astronomical silicate" in spherical grains). We
outline a computationally fast and relatively accurate method that can be used
for a multiwavelength analysis of aggregate dust in protoplanetary disks.
