Effective mass in quantum effects of radiation pressure

We study the quantum effects of radiation pressure in a high-finesse cavity with a mirror coated on a mechanical resonator. We show that the optomecha nical. coupling can be described by an effective susceptibility which takes into account every acoustic modes of the resonator and their coupling to t he light. At low frequency this effective response is similar to a harmonic response with an effective mass smaller than the total mass of the mirror. For a piano-convex resonator the effective mass is related to the light sp ot size and becomes very small for small optical waists, thus enhancing the quantum effects of optomechanical coupling.