Translations:Sketcher ConstrainSnellsLaw/7/en

Remarks

 * The actual Snell's law constraint enforces the plain law equation n1*sin(theta1) = n2*sin(theta2). It needs the line ends to be made coincident and on the interface by other constraints. The necessary helper constraints are added automatically based on the current coordinates of the elements.
 * Python routine does not add the helper constraints. These must be added manually by the script (see example in Scripting section).
 * These helper constraints can be temporarily deleted and the endpoints dragged apart, which can be useful in case one wants to construct a reflected ray or birefringence rays.
 * Unlike the reality, refraction indices are associated with rays of light, but not according to the sides of the boundary. This is useful to emulate birefringence, construct paths of different wavelengths due to refraction, and easily construct angle of onset of total internal reflection.
 * Both rays can be on the same side of the interface, satisfying the constraint equation. This is physical nonsense, unless the ratio n2/n1 is 1.0, in which case the constraint emulates a reflection.
 * Arcs of circle and ellipse are also accepted as rays (physical nonsence).