# How does momentum transfer in light scattering relate to the object size?

The momentum transfer vector is defined as the difference between the incident vector and the scattered vector. In the case of coherent light scattering, the momentum transfer vectors can be constructed in the Ewald's sphere (or in the reciprocal space). Thus, the magnitude of the momentum transfer vector somehow relates the length scale of the scatterer. Why is that? Also, what does it mean by "the scattering amplitude", how is it related to momentum transfer vector magnitude? I'm so confused.