Dust Opacities*

Dust particles are the dominant source of opacity at (almost) all wavelengths and in (almost) all regions of protoplanetary disks. By this they govern the transport of energy through the disk and thus the thermal structure. Furthermore, their spectral properties determine the low resolution spectral signature observed at infrared wavelengths. The infrared resonances that can be observed using low resolution infrared spectroscopy can be used to identify the composition and size distribution of the dust. The opacities depend on the size, shape, structure and composition of the particles, and computing them is not always trivial. In this chapter the ways in which the opacity of dust particles depend on the dust characteristics is discussed. Methods to compute them are outlined and the difficulties that can be encountered are addressed