Optics cornu spiral and diffraction

[lazytofindname]

Hi, i am in second year of university and doing optics
I have a questions in one ofmy assignment :

Fresnel diffraction is observed behind a wire 0.37 mm thick, which is placed 2 m from the light source and 3 m from the observation screen. If the light of wavelength 630 nm is used, find, using Cornu spiral, the irradiance of the diffraction pattern at the screen on the axis. Express the answer as some number times unobstructed irradiance.

The problem is I have no idea out to use the cornu spiral, i looked everywhere online and nobody really explains it
In my textbook they don't really explain it either
I know I'm supposed to put the beginning of my work but i don't know where to start
I think I'm supposed to find the variable u and plot in on the spiral but it doesn't make any sense for me..

 

[sophiecentaur]

Whilst I can't claim to have tried it, the description of using the Cornu Spiral seems to be reasonably clear in this link.
It does seem to show how to work out the resultant vector from all contributions of elements around an edge, using the spiral curve. Hope it helps. At least you could read through it and it may help a bit.
If you are having a difficulty actually scaling the spiral to your case (that could be your problem?) then I suggest you cheat a bit and use the positions of the minima (from simple diffraction theory) to show you where you expect to be on the spiral and that should help.

On the bright side, it's easier nowadays to use the integrals straight and the graphical method is no longer needed, now we have computers with Mathematica!.