  We report on experimental observation of radiation-pressure induced effects
in a high-power optical cavity. These effects play an important role in next
generation gravitational wave (GW) detectors, as well as in quantum
non-demolition (QND) interferometers. We measure the properties of an optical
spring, created by coupling of an intense laser field to the pendulum mode of a
suspended mirror; and also the parametric instability (PI) that arises from the
nonlinear coupling between acoustic modes of the cavity mirrors and the cavity
optical mode. Specifically, we measure an optical rigidity of $K = 3 \times
10^4$ N/m, and PI value $R = 3$.
