Rendering of participating media is an important aspect of every modern renderer. When I say participating media, I am not just talking about fog, fire, and smoke. All matter is composed of atoms, which can be sparsely (e.g. in a gas) or densely (e.g. in a solid) distributed in space. Whether we consider the particle or the wave nature of light, it penetrates all matter (even metals) to a certain degree and interacts with its atoms along the way. The nature and the degree of "participation" depend on the material in question.