  Given any light position S in the complex projective plane P^2 and any
algebraic curve C of P^2 (with any kind of singularities), we consider the
incident lines coming from S (i.e. the lines containing S) and their reflected
lines after reflection off the mirror curve C. The caustic by reflection is the
Zariski clusure of the envelope of these reflected lines. We introduce the
notion of reflected polar curve and express the class of the caustic by
reflection in terms of intersection numbers of C with the reflected polar
curve, thanks to a fundamental lemma established in [14]. This approach enables
us to get an explicit formula for the class of the caustic by reflection in
every case in terms of intersection numbers of the initial curve C.
