Alumina nanopore structures find applications in magnetic sensors, optical filters, and various biological devices. In this work, we present a ray-optics model for the optical filter. We present a detailed simulation and a simplified analytical expression for the reflectance as a function of the alumina parameters such as pore diameter, pore density, alumina thickness, and a function of the wavelength and angle of incidence of the illuminating plane electromagnetic wave. The reflectance vs wavelength in the range of 400-800 nm obtained from the simulation and the analytical expression are compared with that of the experiments for thin and thick alumina. All results agree well for a thin layer of alumina pores (90 nm). When comparing experimental and theoretical results for a thick layer of alumina (1300 nm), the results disagree. One possible explanation for the discrepancy is that the dielectric constant may vary with the thickness of the anodized alumina. Based on this work, a ray optic mixing theory is presented for waves propagating obliquely to parallel slabs of dielectric mediums with a degree of spatial periodicity.