  We investigate the frequency comb induced radiation pressure force acting on
a cloud of cold $^{87}$Rb atoms. Reduction and spectral broadening of the
frequency comb force are observed as the cloud's optical thickness is
increased. Since the radiation pressure force is uniquely determined by light
scattered by an atomic cloud, we discuss different scattering mechanisms, and
point to the shadow effect as the dominant mechanism affecting FC-induced force
in resonantly excited dense atomic clouds. Our results improve the
understanding of the interaction of frequency comb light with many-atom
ensembles, which is essential for novel frequency comb applications in
simultaneous multi-species cooling, multi-mode quantum memories, and multi-mode
atom-light interfaces.
