# Fraunhofer diffraction integra theorem

1. Aug 18, 2012

### pop_ianosd

1. The problem statement, all variables and given/known data
This is not an actual problem, it is a general matter about the equation that describes the Fraunhofer diffraction;
So, for a given aperture(A), which is evenly illuminated from the left(let's say), the Electric Field complex amplitude at a point P(on the right-the screen) can be computed by the formula:

E$_{P}$ = K$\int$$_{A}$e$^{ikr}$dA,
where r is the distance from the surface element dA to the point P.

My questions concerns the fact that I have seen this integral being applied (in a physics book) to non-point surface elements dA, but to line surface elements(dA = Ldy).
The problem I see about this is that one cannot define a distance r between a point and a line-surface element.
The exact case was at calculating the diffraction pattern given by a rectangular (Width>>Height) aperture, and later for a circular aperture.
How is this possible?

2. Relevant equations

3. The attempt at a solution