  Use of resonant light forces opens up a unique approach to high-volume
sorting of microspherical resonators with much higher uniformity of resonances
compared to that in coupled-cavity structures obtained by the best
semiconductor technologies. In this work, the spectral response of the
propulsion forces exerted on polystyrene microspheres near tapered microfibers
is directly observed. The measurements are based on the control of the detuning
between the tunable laser and internal resonances in each sphere with accuracy
higher than the width of the resonances. The measured spectral shape of the
propulsion forces correlates well with the whispering-gallery mode resonances
in the microspheres. The existence of a stable radial trap for the microspheres
propelled along the taper is demonstrated. The giant force peaks observed for
20-{\mu}m spheres are found to be in a good agreement with a model calculation
demonstrating an efficient use of the light momentum for propelling the
microspheres.
