Actually, this is a two-step process which, for convenience, we ``lump'' into one subheading. The two steps are:

- Trace particles to intersection points, and
*Determine the particle's interaction with the surface of interest.*

These steps are displayed pictorially in
Figure 19.
Note that, the particles are ``translated'' along their initial direction
vectors until a surface of intersection is determined (this completely
specifies the geometry of intersection, in particular the angle of incidence
which determines the material properties).
At these angles of incidence, and for the specific materials of the surfaces
of intersection, we ``look up'' the material properties.
We have the following possibilities, so indicated on
Figure 19:
absorbed (**A**),
diffusely reflected ()
or specularly reflected ()
(here, we disallow transmissions for clarity).
For those particles which are absorbed, we count them in the absorption
matrix , where:

Thus, after this event:

Figure 19: Step-2 Particle Disposition.