  We report the observation of the optomechanical strain applied to thermal and
to quantum degenerate $^{87}\text{Rb}$ atomic clouds when illuminated by an
intense, far detuned homogenous laser beam. In this regime the atomic cloud
acts as a lens which focuses the laser beam. As a back action, the atoms
experience a force opposite to the beam deflection, which depends on the atomic
cloud density profile. We experimentally demonstrate the basic features of this
force, distinguishing it from the well-established scattering and dipole
forces. The observed strain saturates, ultimately limiting the momentum impulse
that can be transferred to the atoms. This optomechanical force may effectively
induce inter-particle interactions, which can be optically tuned.
