Instead, they are approximated with a statistical model: Because the particles are very small, and the distance between the particles is much larger than the particle size, individual interactions of light with the particles can be modeled as statistically independent. Of course, the sheer number of these particles makes it infeasible to actually raytrace them individually.
In theory, participating media could therefore be handled by a traditional ray tracer with only surface intersections. These particles are solid objects, and light refracts or reflects off of these objects as it would on a normal surface. The appearance of volumes (also called participating media) in nature is caused by tiny particles, such as dust, water droplets or plankton, that are suspended in the surrounding fluid, such as air or water.
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