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
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
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).