We investigate the applicability of effective medium models for describing the effects of surface roughness on ellipsometric data. The ellipsometric parameters of a model surface composed of truncated spherical bumps on a substrate were calculated and compared to the Bruggeman effective medium theory. It is possible to fit the model calculations with the Bruggeman theory. However, the thickness and volume fraction of the effective medium layer describing the surface roughness do not correspond to the physical height and volume fraction of the bumps on the model surface.