# Physical laws

### Reflection

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, α

_{i}, is egal to the reflecting one, α

_{r}. 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.

### Transmission

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 α

_{i} and refraction α

_{r} are bound by the following formula:

**n**_{i} sin α_{i} = n_{r} sin α_{r}
where n

_{i} is the refraction index of the indicent medium, and n

_{r} the one of the refraction medium.

### Diffraction

A diffraction for a wedge with interior angle of (2- n) π is depicted on the above figure, where
Φ

_{0} 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).