We integrate two computational techniques in order to determine the optical properties of self-assembled mixtures of diblock copolymers and nanoparticles. To determine the morphology of the system, we first use an approach that combines a self-consistent-field theory (SCFT) for the diblocks with a density functional theory (DFT) for the particles. Using this SCF/DFT model, we focus on the lamellar phase of AB diblocks and calculate volume fraction profiles for systems containing selective or nonselective nanoparticles. We use the volume fraction profiles from the SCF/DFT and assign typical dielectric constants for the different polymer domains and the particles to characterize the dielectric properties of the composite. A finite difference time domain (FDTD) technique is then used to simulate the propagation of light through this heterogeneous material. The results of this study allow us to determine how the polymer-particle interactions affect the spatial distribution of fillers within the polymer matrix and how this distribution in turn affects the optical properties of the nanocomposite. The findings can provide guidelines for facilitating the design of photonic band gap materials.