  The structural origin of the first sharp diffraction peak (FSDP) in amorphous
silica is studied by analyzing chemical and radial ordering of silicon (Si) and
oxygen (O) atoms in binary amorphous networks. The study shows that the
chemical order involving Si--O and O--O pairs play a major role in the
formation of the FSDP in amorphous silica. This is supplemented by small
contributions arising from the relatively weak Si--Si correlations in the
Fourier space. A shell-by-shell analysis of the radial correlations between
Si--Si, Si--O and O--O atoms in the network reveals that the position and the
intensity of the FSDP are largely determined by atomic pair correlations
originating from the first two/three radial shells on a length scale of about
5--8 {\AA}, whereas the fine structure of the intensity curve in the vicinity
of the FSDP is perturbatively modified by atomic correlations arising from the
radial shells beyond 8 {\AA}. The study leads to a simple mathematical
relationship between the position of the radial peaks ($r_k$) in the partial
pair-correlation functions and the diffraction peaks ($Q_k$) that can be used
to obtain approximate positions of the FSDP and the principal peak. The results
are complemented by numerical calculations and an accurate semi-analytical
expression for the diffraction intensity obtained from the partial
pair-correlation functions of amorphous silica for a given radial shell.
