Caustic (mathematics)

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For other uses, see Caustic (disambiguation).
Reflective caustic generated from a circle and parallel rays

In differential geometry and geometric optics, a caustic is the envelope of rays either reflected or refracted by a manifold. It is related to the concept of caustics in optics. The ray's source may be a point (called the radiant) or parallel rays from a point at infinity, in which case a direction vector of the rays must be specified.

More generally, especially as applied to symplectic geometry and singularity theory, a caustic is the critical value set of a Lagrangian mapping (πi) : LMB; where i : LM is a Lagrangian immersion of a Lagrangian submanifold L into a symplectic manifold M, and π : MB is a Lagrangian fibration of the symplectic manifold M. The caustic is a subset of the Lagrangian fibration's base space B.[1]


A catacaustic is the reflective case.

With a radiant, it is the evolute of the orthotomic of the radiant.

The planar, parallel-source-rays case: suppose the direction vector is (a,b) and the mirror curve is parametrised as (u(t),v(t)). The normal vector at a point is (-v'(t),u'(t)); the reflection of the direction vector is (normal needs special normalization)

2\mbox{proj}_nd-d=\frac{2n}{\sqrt{n\cdot n}}\frac{n\cdot d}{\sqrt{n\cdot n}}-d=2n\frac{n\cdot d}{n\cdot n}-d=\frac{

Having components of found reflected vector treat it as a tangent


Using the simplest envelope form

F(x,y,t)=(x-u)(bu'^2-2au'v'-bv'^2)-(y-v)(av'^2-2bu'v'-au'^2) =x(bu'^2-2au'v'-bv'^2)
+b( u'v'^2 +2uv'v'' -u'^3 -2uu'u'' -2u'v'^2 -2u''vv' -2u'vv'')
+a(-v'u'^2 -2vu'u'' +v'^3 +2vv'v'' +2v'u'^2 +2v''uu' +2v'uu'')

which may be unaesthetic, but F=F_t=0 gives a linear system in (x,y) and so it is elementary to obtain a parametrisation of the catacaustic. Cramer's rule would serve.


Let the direction vector be (0,1) and the mirror be (t,t^2). Then

u'=1   u''=0   v'=2t   v''=2   a=0   b=1

and F=F_t=0 has solution (0,1/4); i.e., light entering a parabolic mirror parallel to its axis is reflected through the focus.


  1. ^ Arnold, V. I.; Varchenko, A. N.; Gusein-Zade, S. M. (1985). The Classification of Critical Points, Caustics and Wave Fronts: Singularities of Differentiable Maps, Vol 1. Birkhäuser. ISBN 0-8176-3187-9. 

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