  After recalling the notion of caustics of plane curves and basic equations,
we first show the birationality of the caustic map for a general source point S
in the plane. Then we prove more generally a theorem for curves D in the
projective space of 3x3 symmetric matrices B. For a general 3x1 vector S the
projection to the plane given by B --- BS is birational on D, unless D is not a
line and D is contained in a plane of the form Delta_v = {B | Bv = 0}.

Algebraic Geometry;Differential Geometry