Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.