Domain Selection
It is possible for rays to pass through domains in the geometry and to propagate in the void region outside these domains. Boundary conditions can be specified at any boundary, including boundaries that are not adjacent to any domain in the geometry. This means that rays can be reflected or absorbed by a surface in 3D, or a line segment in 2D, even if it is not attached to any other object. The ray tracing algorithm can also detect boundary interactions in any order, without this order being specified.
Usually, the domain selection for the Geometrical Optics interface should include all objects that the rays might pass through. In a lens system, this would mean that all lenses are included but not necessarily the mount for these lenses.
Domains that are not included in the selection for the physics interface do not need to be meshed. However, these domains would still require a mesh if some other variables, like displacement and temperature, are solved for there. See Meshing Guidelines for more details.
Some physics features require a domain mesh and will not function on domains outside the physics interface selection. This includes all types of Accumulator (Domain) feature, such as the dedicated Ray Heat Source multiphysics feature.