Abstract
Given a point S (the light position) in ℙ3 and an algebraic surface 𝓩 (the mirror) of ℙ3, the caustic by reflection ΣS(𝓩) of 𝓩 from S is the Zariski closure of the envelope of the reflected lines 𝓡m obtained by reflection of (Sm) on 𝓩 at m ∈ 𝓩. We use the ramification method to identify ΣS(𝓩) with the Zariski closure of the image, by a rational map, of an algebraic 2-covering space of 𝓩. We also give a general formula for the degree (with multiplicity) of caustics (by reflection) of algebraic surfaces of ℙ3.
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