Title: Optical properties and radiative transferLecturers: Ciska Kemper & Alvaro Sanchez-MongeContext: ICE summer school 2023 -- Life Cycle of DustDate: 4 July 2023 -- 14:00-18:00 Optical properties In order to calculate the absorption and scattering efficiencies, we will be using a software package called optool[^optool1][^optool2][^optool3][^optool4], available from https://github.com/cdominik/optool, and developed by Carsten Dominik, based on the routines written by Michiel Min and Ryo Tazaki. This package allows the conversion from $n$ and $k$ values to absorption and scattering efficiencies (including a full scattering matrix) for different grain shapes, including Mie theory, hollow spheres, fractal grains, and continuous distribution of ellipsoids. It also allows for the combination of different materials in one grain, including vacuum, to represent porosity. optool calculates the mass absorption coefficients $\kappa$, rather than $Q$, as this is more meaningful for the general, e.g. non-spherical case. $\kappa$ gives the extinction cross-section (in cm$^2$) per mass (in g) of material. Derive that for the spherical case, $\kappa$ and $Q$ are related as: $$ \kappa = \frac{3}{4} \frac{1}{\rho_\mathrm{d}} \frac{Q}{a} $$