# Caustic (mathematics)

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]

## Catacaustic

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 ${\displaystyle (a,b)}$ and the mirror curve is parametrised as ${\displaystyle (u(t),v(t))}$. The normal vector at a point is ${\displaystyle (-v'(t),u'(t))}$; the reflection of the direction vector is (normal needs special normalization)

${\displaystyle 2{\mbox{proj}}_{n}d-d={\frac {2n}{\sqrt {n\cdot n}}}{\frac {n\cdot d}{\sqrt {n\cdot n}}}-d=2n{\frac {n\cdot d}{n\cdot n}}-d={\frac {(av'^{2}-2bu'v'-au'^{2},bu'^{2}-2au'v'-bv'^{2})}{v'^{2}+u'^{2}}}}$

Having components of found reflected vector treat it as a tangent

${\displaystyle (x-u)(bu'^{2}-2au'v'-bv'^{2})=(y-v)(av'^{2}-2bu'v'-au'^{2}).}$

Using the simplest envelope form

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

which may be unaesthetic, but ${\displaystyle F=F_{t}=0}$ gives a linear system in ${\displaystyle (x,y)}$ and so it is elementary to obtain a parametrisation of the catacaustic. Cramer's rule would serve.

### Example

Let the direction vector be (0,1) and the mirror be ${\displaystyle (t,t^{2}).}$ Then

${\displaystyle u'=1}$   ${\displaystyle u''=0}$   ${\displaystyle v'=2t}$   ${\displaystyle v''=2}$   ${\displaystyle a=0}$   ${\displaystyle b=1}$
${\displaystyle F(x,y,t)=(x-t)(1-4t^{2})+4t(y-t^{2})=x(1-4t^{2})+4ty-t}$
${\displaystyle F_{t}(x,y,t)=-8tx+4y-1}$

and ${\displaystyle F=F_{t}=0}$ has solution ${\displaystyle (0,1/4)}$; i.e., light entering a parabolic mirror parallel to its axis is reflected through the focus.

## References

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.