We consider a polymer as a chain of monomers, each with its position vector  and scattering amplitude . For simplicity, it is worthwhile considering identical monomers in the chain, such that all .
An incoming ray (of light/neutrons/X-ray etc.) has a wave vector (or momentum) , and is scattered by the polymer to the vector . This enables us to define the scattering vector . 
By coherently summing the contributions of all  monomers, we get the scattering intensity from a single polymer, as a function of :