Talk:Fraunhofer diffraction

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I keep getting contradictions on Wikipedia:

This page:

"...exceeding that defined as fresnel diffraction (F >> 1)"

"...any wave which has a fresnel number of larger than one -- F >> 1, is therefore subject to Fraunhofer diffraction"

Fresnel diffraction:

"Fresnel diffraction or near-field diffraction is the diffraction pattern of an electromagnetic wave obtained close to the diffracting object (often a source or aperture). More accurately, it is the diffraction case when the Fresnel number is large and thus the Fraunhofer approximation (diffraction of parallel beams) can not be used."

Fresnel number:

"Depending on the value of F the diffraction theory can be simplified into two special cases:

  • Fresnel diffraction for F <or= 1
  • Fraunhofer diffraction for F >> 1"

Having just done experiments on Fraunhofer diffraction I was pretty sure it occurs when L is large, which makes the Fresnel number small, ie F < 1 , however I was pretty confused about the topic. I'd appreciate it if someone could clear this up a bit? This is probably the first time when Wikipedia has made me even more confused.

Apologies[edit]

Apologies, i wrote the fresnel case in here. Cheers for changing it over; i didn't notice until i came back just now. SConfident.gif J O R D A N [talk ] 12:39, 7 February 2007 (UTC)

History?[edit]

The history of the wave theory of light refers mostly to Grimaldi, Huygens, Young and Fresnel. However we call single slit diffraction "Fraunhofer diffraction". I have looked through Fraunhofer article on wikipedia but I fail to see where Fraunhofer fits in the history of diffraction and what he did to deserve the honor of having single slit diffraction named after him. —The preceding unsigned comment was added by Pboudreau (talkcontribs) 23:44, 22 April 2007 (UTC).

Well, Fraunhofer did create the diffraction grating for the first accurate spectroscopy. It's reasonable to assume he observed Fraunhofer diffraction in sharp spectral lines (you pass the light through a slit before shining it on the diffraction grating) and found that the same far-field math can explain both phenomena. That's all a guess, though. — Laura Scudder 03:11, 23 April 2007 (UTC)

How can an aperture have a gaussian profile?[edit]

Please, enlighten me. 129.173.133.235 13:48, 30 October 2007 (UTC)

For example a dia film with a transmittance varying with position. Han-Kwang (t) 17:20, 30 October 2007 (UTC)

Change over to Goodman[edit]

I think the treatment of Fraunhofer diffraction is much more understandable using the methods of Fourier analysis. The fourier treatment in Goodman is excelent. I plan on switching this article over to proper analysis in the near future. Gfutia (talk) 18:12, 13 March 2008 (UTC)

I agree that the content needs some updating. Like it is now, the explanation sounds a bit 19th century. I believe less importance should be put on slits and apertures. Fraunhoffer diffraction occurs with any wavefield distribution, not only these simple binary masks. According to me, the Fourier transform property should be mentioned in the very first paragraph.Pierre T. (talk) 21:23, 17 March 2008 (UTC)
I added the Fraunhofer formula from Goodman. I know it's not explained at all, but I just wanted to get it in there. I'm a graduate student taking a course in Fourier Optics, something that largely discusses the Frunhofer regime. Time permitting, I'm going to try to update this article to make it more accessible. Gfutia (talk) 03:29, 24 March 2008 (UTC)
I suggest that both treatments should be provided. I have defined the variables in the equation provided and removed the constant term as I don't think it aids understanding. I notice that all the comments here date from 2008, so maybe all that was promised has been done and undone again???? Epzcaw (talk) 10:39, 16 May 2011 (UTC)

Why a photo of Smith on a wanted poster for Jones?[edit]

The article gives a diagram showing Fresnel diffraction, but gives no diagram showing Fraunhofer diffraction. The Fresnel diffraction diagram does not serve any explanatory purpose I can see. Surely the average well-informed reader will not get any useful idea of what a Fresnel diffraction pattern looks like from this highly abstract chart. P0M (talk) 07:29, 2 March 2009 (UTC)

Agreed. I am trying to think, and then make, an alternative. Any suggestions? Epzcaw (talk) 10:36, 16 May 2011 (UTC)

Decided to remove it as no-one has objected. Epzcaw (talk) 17:29, 7 June 2011 (UTC)

Only the size?[edit]

"...causing only the size of an observed aperture image to change..." As opposed to what else changing? The possibilities can't be inferred without prior knowledge about optics. ᛭ LokiClock (talk) 14:50, 26 April 2011 (UTC)

I have simplifed the introduction, taking previous comments on board. I removed the phrase "is a form of wave diffraction that occurs" since it is a model of what occurs, not what actually occurs. I also removed the phrase "only the size of an observed aperture image to change[1][2] due to the far-field location of observation and the increasingly planar nature of outgoing diffracted waves passing through the aperture.". This is not correct. Fraunhofer (or indeed any other) diffraction changes not just the size, but also the intensity distribution of light to change as well. A uniformly illuminated slit become a series of stripes of decreasing intensity on either side, a uniformly illuminated circle become an Airy Disk, etc, etc. Epzcaw (talk) 10:35, 16 May 2011 (UTC)

Major re-write[edit]

I think this article needs a major re-structuring and re-writing. The subsection "Form" and its subsections are disjointed, partly repetitive, and in some cases, wrong. There is too much discussion of Fresnel diffraction which is covered in another article.

Not sure what this sentence meams:

":a wave is split into several outgoing waves when passed through an aperture, slit or hole, and is usually described through the use of observational experiments using lenses to purposefully diffract light. "

Light is not split into 'several outgoing waves' when it passes through an aperture. It does exactly the same as when it doesn't pass thorugh an aperture, which is to effectively act as an infinite set of point sources all producing spherical wavelets which all add togheter.

Or this:

"When waves pass through, the wave is split into two diffracted waves traveling at parallel angles to each other along with the continuing incoming wave"

Pass through where? What two diffracted waves?

The section called 'Apreture form' doesn't seem to have anything to do with aperture form and again there is unnecessary discussion of Fresnel diffraction. A new page called, say, 'Fresnel and Fraunhofer diffraction compared' might be useful, and some of this material could go there.

The 'Slit form' section is out of place here - perhaps should be in a section about practical aspects of diffraction.

I hope to add a new section showing how the Fraunhofer diffraction equation is derived from the Kirchhoff diffraction equation, which is a mathematically based version of the empirical Huygens-Fresnle equation. This should alos explain that the Fraunhofer diffraction equation is the Fourier transform of the aperture function.

The last section in 'Forms', 'Amplitude transmittance' could go in that section.

I would welcome comments on this if/before I embark on a major re-write. Epzcaw (talk) 17:35, 16 May 2011 (UTC)

I have now re-written the Fraunhofer diffraction page, and created a second page called "Fraunhofer diffraction (mathematics).
The first explains Fraunhofer diffraction mainly qualitatively, with a few seme-quantitative derviations showing how a specific diffraction pattern can be explained. All the diagrams are from Wiki Commons, but in each case, I have provided a reference to a similar diagram in an optics textbook.
The second article give the Fraunbofer diffraction equation in various mathematical forms, and also gives calculations for various forms of aperture, using both integration and Fourier transforms.
I have used six standard optics text books as sources, but have done the calculations in a single unified form. Again, in each case, I have provided a reference to a similar calculation.
The second article in not quite finished, but I hope to complete in the next few days.
They can be found at:
User:Epzcaw/Fraunhofer diffraction
User:Epzcaw/Fraunhofer diffraction (mathematics)
Comments and constructive criticism welcome. Epzcaw (talk) 18:06, 7 August 2011 (UTC)
New page subsitituted for previous Fraunhofer diffraction.
New page Fraunhofer diffraction (mathematics) created.
Epzcaw (talk) 10:21, 9 August 2011 (UTC)