  We develop the idea of manipulating the scattering length $a$ in
low-temperature atomic gases by using nearly resonant light. As found, if the
incident light is close to resonance with one of the bound $p$ levels of
electronically excited molecule, then virtual radiative transitions of a pair
of interacting atoms to this level can significantly change the value and even
reverse the sign of $a$. The decay of the gas due to photon recoil, resulting
from the scattering of light by single atoms, and due to photoassociation can
be minimized by selecting the frequency detuning and the Rabi frequency. Our
calculations show the feasibility of optical manipulations of trapped Bose
condensates through a light-induced change in the mean field interaction
between atoms, which is illustrated for $^7$Li.
