  The dynamic spectra of pulsars frequently exhibit diverse interference
patterns, often associated with parabolic arcs in the Fourier-transformed
(secondary) spectra. Our approach differs from previous ones in two ways:
first, we extend beyond the traditional Fresnel-Kirchhoff method by using the
Green's function of the Helmholtz equation, i.e.\ we consider spherical waves
originating from three dimensional space, not from a two dimensional screen.
Secondly, the discrete structures observed in the secondary spectrum result
from discrete scatterer configurations, namely plasma concentrations in the
interstellar medium, and not from the selection of points by the stationary
phase approximation. Through advanced numerical techniques, we model both the
dynamic and secondary spectra, providing a comprehensive framework that
describes all components of the latter spectra in terms of physical quantities.
Additionally, we provide a thorough analytical explanation of the secondary
spectrum.
