Talk:Fresnel diffraction


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fresnel number and approximation
On the page about fresnel's number, it says that this equation is only accurate when its greater than or equal to 1. Is this correct? On the page about diffraction, it says that it describes the more general case of diffraction when compared to far-field diffraction. These are contradictory, and this page only says its used when the wave is "a small number of wavelengths away from the diffracting object". Is it an approximation or genralization? Fresheneesz 06:02, 20 March 2006 (UTC)

In fact, the Fresnel approximation should be good whenever $$\frac{N_F \theta_{\rm{max}}^2}{4}\ll 1$$. $$N_F$$ can be smaller than 1, but doesn't have to like in the far-field approximation. --Ahmes 14:37, 3 July 2006 (UTC)

Alternative Forms: Fourier Transform
I do not agree with the following section, which states that H is the fouriertransform of h


 * $$= H(p,q) \cdot G(p,q) |_{p = {x \over \lambda z}, q = { y \over \lambda z } }$$

where
 * $$H(p,q) = \mathcal{F} \left\{ h(x,y) \right\} $$

This can not be true, since H is equal to h. —Preceding unsigned comment added by 130.75.103.87 (talk) 10:11, 2 February 2009 (UTC)

Undefined Values, unclear Fourier Transform
In this text


 * $$G(p,q) = \mathcal{F} \left\{ g(x,y) \right\} \equiv \iint_{-\infty}^{\infty} g(x,y) e^{-i 2 \pi (p x + q y)} dx dy $$

where p and q are spatial frequencies (in units of lines/meter). The Fresnel integral can be expressed as:


 * $$E(x,y,z) = \frac{e^{i k z}}{i \lambda z} e^{i \frac{\pi}{\lambda z}(x^2 + y^2)} \mathcal{F}

\left. \left\{ E(x',y',0) e^{i \frac{\pi}{\lambda z} (x'^2 + y'^2)} \right\} \right|_{p = \frac{x}{\lambda z}; q = \frac{y}{\lambda z}} $$

g(x,y) is used without any definition. Also it is not clear why the FT is done on
 * $$E(x',y',0) e^{i \frac{\pi}{\lambda z} (x'^2 + y'^2)}$$ —Preceding unsigned comment added by 130.75.103.87 (talk) 10:28, 2 February 2009 (UTC)

Photos
The picture given as an example for fresnel diffraction should be changed, because it gives you a wrong idea about the phenomenon. 80.153.126.37 (talk) 13:22, 29 May 2009 (UTC)
 * What do you mean? It is consistent with a whole page full of fresnel diffraction photos given by Francis Weston Sears in his ''Optics," which is a standard text. It is not as nice a photo. If anyone has access to a better photo that shows a center black spot, or a black spot and black rings, let them supply the better picture. If you don't believe Sears (even though Sears and Zemansky is still a standard physics text), look at the simulation software that is linked to the article.P0M (talk) 07:48, 30 May 2009 (UTC)
 * OK, i took some shots and i'll try to upload them now. 80.153.126.37 (talk) 20:49, 14 July 2009 (UTC)
 * I had no tripod, so the picture is a bit unsharp, i will do better ones in 3 weeks. —Preceding unsigned comment added by 80.153.126.37 (talk) 20:53, 14 July 2009 (UTC)

Why is the picture i uploaded not visible any more? Always a beginner (talk) —Preceding undated comment added 14:21, 7 October 2009 (UTC).
 * What often happens is that somebody uploads a photo but does not give information about it. Specifically you need to state that it is your own work (by putting your user name in as "author," for instance), and you need to choose a version of the permission to reproduce from the GFDL drop-down menu. One of them is marked as recommended, so I usually choose that one as default. So all you really are doing is to say that you made the photo and that you give it away. Also it is best to give a description, i.e., say that it is a pattern produced by fresnel diffraction, the hole is .3 mm in diameter (or whatever it actually was), etc.  In other words you supply all the technical stuff that is relevant to the physics of the phenomenon in question.
 * If I fail to fill in required information I generally get a warning on my user page telling me the photo will be deleted if I do not fix things soon. Since you are not a registered user, you probably never saw any notice, and so the administrators saw a photo that was possibly a copyrighted image copied from someplace without permission, and they deleted it. P0M (talk) 22:59, 7 October 2009 (UTC)

Variable Definition
Many variables appear on this page, but not all of them seem to be defined. Some of the variables I have noticed include "a" and "L".

1/R^2 Dependency of field instead of 1/R
In the general form of the integral for the field at a given point, the expression involves a 1/R^2 dependency of the field. It should actually be a 1/R dependency on distance. see here for derivation: http://www.cv.nrao.edu/course/astr534/PDFnewfiles/LarmorRad.pdf I edited it. Please tell me if I'm doing things inappropriately as I am new to editing wikipedia. Thanks! Edit: just realized that the page is dealing with the near field diffraction pattern; different sources call the Fresnel pattern differently; some call it the far field and others call it the near field pattern; that's where the 1/r^2 difference came from. edited back. — Preceding unsigned comment added by Buhbuhbuhbowser (talk • contribs) 23:13, 2 May 2012 (UTC)

I'm with you on thinking this should be 1/r...? Why wouldn't the 1/r equation apply in the near field too? See for example http://www.iue.tuwien.ac.at/phd/kirchauer/node50.html eqn 4.4 Pwrichards (talk) 21:40, 11 May 2016 (UTC)

It should be 1/r, compare it just to the last equation in the paragraph, there it it also 1/r (or 1/z). See also equation 3.1 in Schnars, U. & Jüptner, W. P. O. Digital recording and numerical reconstruction of holograms. Meas. Sci. Technol. 13, R85 (2002). Low87 (talk) 10:59, 1 July 2016 (UTC)