In this section, we address the physical aspects involved in Monte Carlo particle/surface interactions. All physics involved are embodied in the material properties of the surfaces in the enclosure. There are two physical aspects to the material properties: (1) emission and (2) particle/surface interactions. Both aspects must be modelled with sufficient accuracy to achieve the fidelity required to represent the problem.

When emitting, we must replicate: (1) the directional distribution of physical
emissions from a point on the surface, (2) the spatial distribution of
emissions over a surface, and (3) other distributions of interest, e.g. the
energy distribution.
We proceed assuming that all distributions are independent, i.e., not
interrelated (e.g., the directional distribution is the same, no matter where
the emission point is).
In fact, each * numerical* particle represents an * assemblage* of
physical particles.
``Enough'' numerical particles must be emitted and traced to extinction in
order to achieve convergence (this is similar to refining the grid in
continuum calculations).

Particle/surface interactions are defined simply in terms of probabilities of certain physical behaviors upon interaction. This is fairly straightforward if the particle is absorbed by the surface. It becomes complicated if the particle survives, and an outgoing direction must be chosen. Because good data for particle/surface interactions are not typically available, it is common practice to make approximations in this regard. Fortunately, the approximations are representative of many physical situations (here, nature is in harmony with our modelling capability).