We report on nano-scale optical effects of amorphous silicon layer conformally deposited on randomly textured zinc oxide layers on glass substrates investigated by near-field scanning microscopy. Such textured layers are used in thin-film photovoltaic devices to enhance light trapping. Experimental results are compared to theoretical data, obtained from large scale finite-difference time-domain simulations. Light localization on the surface of the textured interface and a focusing of light by the structure further away are observed. The measurements are compared with simulations, which provide additional insight into the light intensity distribution inside the solar cell on a nm-scale. It will be shown how this information can be used to optimize light trapping in thin-film solar cells using an amorphous silicon solar cell as an example.