Physical laws



        As in geometrial optics, we can consider a first homogenous and isotropic medium containing two points A and B, and a second opaque medium where no light can be propagated (cf. the figure above). The wave going from A along an incident ray is reflected (or bounced) on the plane P following the reflected ray to B. The incident angle, αi, is egal to the reflecting one, αr. R is the intersection point between the plane P and the segment of line AB', where B' is the symetrical of B with respect to P.



        Let us consider from now, two transparent medium and again two points A and B (see the figure above). The incident ray going from A is deflected (or refracted) at the point R of the plane P to B, following the Snell law:
1. A belongs to the indicent medium and B thre refracted one;
2. the angles of incidence αi and refraction αr are bound by the following formula:
                ni sin αi = nr sin αr
where ni is the refraction index of the indicent medium, and nr the one of the refraction medium.



        A diffraction for a wedge with interior angle of (2- n) π is depicted on the above figure, where Φ0 is the incident angle and Φ the diffraction one.
        The diffracted ray is calculated according to the Uniform Theory of Diffraction.
        The wave path follows an extremum in theory, and a minimum in practice. Then, the diffracted wave arrive at destination according to the smallest path, but going through the diffraction point located on a surface discontinuity (generally an edge).