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.