  Trapped surface formation in general relativity can be studied through a
coupled set of nonlinear equations, where various terms can be neglected, as
was proved by a rigorous mathematical analysis of Christodoulou. This paper is
devoted to a pedagogical synthesis of the mathematical formalism employed in
this analysis, i.e. the optical structure of general relativity investigated by
Christodoulou, Klainerman and other authors after them.
