Ag nanoparticles deposited and grown photocatalytically for a sufficient period of time on a TiO₂ thin film scatter blue light and reflect reddish light when the particles are irradiated directly (i.e. front incidence). On the other hand, they scatter reddish light and reflect blue light when the particles are irradiated through the TiO₂ film (i.e. back incidence). The asymmetric optical behaviour is attributed to the asymmetric plasmon resonance, for which a sufficient height and a sufficient width at the top and the bottom of the Ag nanoparticles are important.