Porous media such as tissues, soil materials, porous rocks, and concrete become darker with increasing water content. Most methods used to quantify this behavior are based on statistical models. Physical models have rarely been used for this purpose. In this study, we implemented a radiative transfer model to analyze the measured reflectance and transmittance of prepared moist soil slabs, accounting for the physical processes of light reflection, refraction, and absorption. We calculated the trace of numerous beams in the image of vertical slab cross sections. This image represents the spatial distribution of the solid, water, and air phase in the soil slab. We implemented this model to test how the arrangement and optical properties of the water phase influences the reflectance. To validate the model assumptions, we qualitatively compared the calculated reflectance and transmittance with measurements. We concluded that reflectance and transmittance depend not only on the amount of water but also on the spatial distribution of water within the pore space of the porous medium.