Light trapping in a waveguide configuration consisting of a thin planar film of hydrogenated amorphous silicon (aSi:H) on a planar silver backreflector is studied theoretically and experimentally. Light trapping is achieved by scattering of light from a silver or silicon nanostrip placed directly on the silicon-film surface. For thin films it is appropriate to think of light trapping in terms of coupling into the guided modes of the air-aSi:H-silver waveguide configuration, which is the focus of this paper. Using the Green’s function surface integral equation method we calculate cross sections governing extinction, out-of-plane scattering, and scattering into guided modes. It is found for geometries with aSi:H-film thicknesses in the range of 50–500 nm that distinct peaks in extinction and scattering cross-section spectra are located at wavelengths determined by the cutoff wavelengths of guided modes, and the wavelengths of those peaks are insensitive to the precise geometry of the scatterer. Measurements of extinction and scattering from an array of silver strips spaced by a large distance of 10 μm on the surface of a geometry with a 290 nm aSi:H-film are found to be in good agreement with theoretical predictions.