Hi, I'm interested in reflection of both particles and light off surfaces. My problem is associated with a model that I'm working on and can be divided in to two parts as far as I see it. The first problem is the reflection of particles off the surface and the change in velocity due to energy losses associated with the reflection or bounce itself. I'm trying to model rigid particles bouncing off a sedimentary surface, e.g., mud, sand, gravel, or rock, but have had difficulty finding any means of calculating coefficients of restitution for different surfaces using known physical properties of the materials or even simply values that have been obtained experimentally. Does anyone know of any means of modelling this kind of process acurately? The second issue that I'm having is modeling the reflection of light off a rough surface. I understand that if light reflects off a surface that is smooth then the angle of incidence is the same as the angle of reflection and everything is very easy. However, if the surface is not smooth, e.g., sandpaper, then for one beam of light hitting the surface at a given angle of incidence there must be a range of possible angles of reflection due to the range of possible orientations of the surface due to the roughness elements. Is anyone aware of a means of connecting the roughness of a surface to the range of possible reflection angles? I imagine that this range of angles could be characterised by some sort of distribution function like a normal distribution where for a given angle of incidence there are angles of reflection that are more common than others, hence the distribution. Thanks.