Talk:Diffraction

Diffractive Optics

"Diffractive Optics" redirects to this article. Diffractive Optics is a technique for making smaller and lighter lenses. It has nothing whatsover to do with the content of this article. 86.147.237.233 (talk) 12:40, 17 March 2012 (UTC)

Actually, these lighter lenses you refer to work by the principles of diffraction described in this article. But if you'd like to start an article on diffractive optics, that would probably be OK, too. Dicklyon (talk) 18:46, 17 March 2012 (UTC)

Are you sure about that? I was under the impression that "Diffractive Optics" was just a Canon marketing term for Fresnel lenses which don't actually work by diffraction. The DO lenses contain Fresnel lenses. Even so, the article doesn't mention the lens construction so my point stands. 86.147.237.233 (talk) 14:32, 22 March 2012 (UTC)

I have seen reference to diffractive optics being a Canon marketing term in several reviews. Diffraction in a camera lens is a bad thing to be avoided where possible. Every camera lens suffers from the degrading effects of diffraction to some extent. The light travelling through a lens always diffracts around the aperture blades softening the image to some extent. As the aperture gets smaller, the diffracted light becomes a larger proportion of the total light throughput detracting from the sharpness of the image. In general, lenses usually give their sharpest image somewhere around f/8. When lenses are constructed out of Fresnel lenses, the steps between the lens segments introduces several further edges around which light diffracts, thus diffractive optic lenses have a lower optical performance. 109.145.22.224 (talk) 12:31, 5 April 2012 (UTC)

Propagation of a laser beam

In this section, it is mentioned "Hence, the smaller the output beam, the quicker it diverges". Here, does "smaller" mean a smaller size or something else? Hopefully someone can help me with this question, thanks! AlexHe34 (talk) 15:14, 30 March 2012 (UTC)

When light is incident on an aperture, the angle into which it is diffracted is inversely proportional to the size of the aperture. One can view the output beam of a laser as being defined by the output aperture of the system. This is much smaller in a semiconductor laer than in, say a He-Ne laser, hence the semiconductor beam spreads out much more than a He-Ne one. It can, of course be collimated (converted into a beam which spreads only very slowly), as in a laser pointer, by using a lens.Epzcaw (talk) 20:50, 22 July 2012 (UTC)

I have fixed few other things here. Laser mirror is a resonator, not an aperture. And the laser beam is not always a fundamental mode (in fact, almost never). — Preceding unsigned comment added by Khrapkorr (talk • contribs) 20:30, 10 February 2015 (UTC)

Double-slit diffraction

You write: "Thomas Young performed a celebrated experiment in 1803 demonstrating interference from two closely spaced slits.[10]"

If you actually read the Bakerian lecture you reference [10] you will see that he does not discuss the celebrated double-slit experiment, nor is the diagram you print in that lecture. the only place Young mentions the experiments is in his popular lectures before Royal Institution (pub. 1807). The diagram is in fact from his lecture on hydraulics, not light, and although he describes a two-slit experiment, he never claims he actually did it. Furthermore the wavelengths he quotes for the wavelengths of light is from the Newton's rings experiment. There is no clear evidence that Young ever performed the experiment.

Tony Rothman

explain me the double slit diffraction (Radhika thakur (talk) 15:19, 21 April 2012 (UTC))

See Double-slit experiment. Also see the comment in this (Diffraction) article about the difference between interference and diffraction. Epzcaw (talk) 07:52, 13 July 2012 (UTC)

Connection to the Heisenberg Uncertainty Principle?

I'm posting this here with the hope that a definitive answer could be included in an amended version of the main article. It was once explained to me in class when I was an undergraduate that single-slit diffraction could be understood in terms of the Heisenberg uncertainty principle. The argument goes as follows: Imagine an incident plane wave of light at wavelength "λ" approaching a 1-dimensional slit aperture from the left, which I will define as the "x" direction. The slit is of height "d" along the "y" direction, and for simplicity's sake is infinite along the z direction. Light must pass through the slit in the form of photons, and the Heisenberg uncertainty principle places limits on these photons' momenta. Specifically, we know that as a photon passes through the slit, its vertical position is known to within an uncertainty "d/2," so the vertical component of momentum, "p_y", is only defined to within a precision of hbar / d.

Now, because the incident light was coming in with wavelength λ, we know that each photon’s total momentum must be 2 π hbar / λ. We can combine the two relationships to give the following relation,

$2\pi d\sin \theta =\lambda .$

To within some factors of π and 2 that can probably be explained away by the specific geometry of the slit, this is exactly the width of the central peak in Fraunhofer diffraction.

My question is this: What happens when the slit becomes narrower than lambda / (2 pi)? That is to say, what happens when the slit becomes so narrow that a photon passing though it must acquire an uncertainty in transverse momentum which is greater than the photon’s total momentum was to begin with? Two possibilities come to mind:

(a) When the slit gets that narrow, light simply cannot get through. This seems reasonable except for the fact that once you close down the slit even a single photon that sneaks through the slit would have to violate the uncertainty principle. Transmittance that is identically zero seems difficult to swallow.

(b) When the slit becomes very narrow, it begins to resemble a cavity where photons can be up-converted to higher energy and momentum. Would this mean that forcing light to pass through a narrow slit changes its color?

Thanks in advance for any thoughts. Csmallw (talk) 04:57, 29 June 2013 (UTC)

Offering a MUCH better example of diffraction - Don Quixote's Windmill.

When you look at "point-like" lights through a rectangular grid, you see four diffraction spikes around each light. This is called a Don Quixote's Windmill, and that should be a heading that re-directs to this page. The best and most common example of this is looking at outside street lighting through an insect screened window or door at night time. Amazingly perceptive yet totally simple spectrographic analysis can be performed just from this phenomenon alone. For example, the difference between incandescent and metal-vapour emission lighting (such as sodium)is immediately obvious. Other examples are looking through lace or sheer curtains. I am happy to take and post some images and do a brief write-up of all of this, but someone else would have to do the redirect. 121.216.26.225 (talk) 09:54, 9 November 2014 (UTC)

The phenomenon you describe is indeed a nice everyday example of diffraction, so images of that be welcome. As for its name, a quick search on Google Books didn't turn up a clear reference for "light diffraction Don Quixote". Fgnievinski (talk) 14:11, 9 November 2014 (UTC)

OK, then, will do. My WIkiactivity is very erratice so it will take some time. — Preceding unsigned comment added by 121.216.207.197 (talk) 10:52, 10 November 2014 (UTC)

I am bewildered. Iam familiar with this effect but am new to the name. I came looking for an explanation of how a Bahtinov mask works. But Bahtinov masks and lace curtains do not have sizes that approach the wavelengths of the (generally) incoherent light affected, they are just repeating patterns - so how can diffraction occur with patterns at such macro scales? The article needs to explain this as it is a phenomenon the article suggests can't happen. Stub Mandrel (talk) 09:10, 19 May 2015 (UTC)

I've removed it from the article for now, given the item you mention is a redlink, we don't have any cite to meet the verifiability policy for it, and considering the concern raised here that it seems to contradict the other cited content. DMacks (talk) 14:12, 19 May 2015 (UTC)

"... Diffraction in the atmosphere by small particles can cause a bright ring to be visible around a bright light source like the sun or the moon. ...

Err, no these bright rings are caused by atmospheric REFRACTION. If you follow the link it even warns you not to confuse the two. 121.216.26.225 (talk) 09:58, 9 November 2014 (UTC)

When the particles are small and, accordingly, the mechanism ambiguous, we really ought to use the word "scattering" rather than "diffraction", "refraction", or "reflection". In this case, if the particles are opaque then "refraction" is definitely not the right word for what is happening.2001:480:91:3304:0:0:0:658 (talk) 19:38, 25 June 2021 (UTC)

Diffraction Pattern of red Laser beam

This image on the front page is named as "Laser Interference" by the uploader. Could the author actually upload a interference pattern rather than diffraction pattern? As far as I know, it's possible to produce such pattern with Michelson and Morley setup. — Preceding unsigned comment added by Mywtfmp3 (talk • contribs) 07:04, 8 September 2015 (UTC)

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Not really geometric shadow

@Johnjbarton, I think your addition of "geometric shadow" is not really correct. As one simple case, consider the focussing of an electron beam as it goes through a single atom (or a string). At least close to the atom you get increased probability. (John Cowley actually tried to get this to work for super-resolution before Cs-correction became possible.) I think there is a comparable effect for an aperature with any type of wave. Please change the lead back, bending is really more appropriate. Ldm1954 (talk) 21:16, 31 December 2024 (UTC)

The previous lede was:

• Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.

This sentence placed "interference" equivalent (or) to "bending of waves" which can't be right.

My version:

• Diffraction is the deviation of waves from straight-line propagation due to an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.

Maybe clearer would be

• Diffraction is the deviation of waves from straight-line propagation, due to an obstacle or through an aperture, into the region of geometrical shadow of the obstacle/aperture.

So my edit did not add geometric shadow but I left it and it seems ok to me. My edit was based on Hecht's first paragraph on Diffraction:

• deviation of light from rectilinear propagation...the effect is a general characteristic of wave phenomena occurring whenever a portion of a wavefront, be it sound, a matter wave, or light, is obstructed in some way. This is similar to "bend" without using that word. "Bending of waves" sounds exactly like "refraction" to me.

Having "geometrical shadow" in here makes sense to me in that the optics divides in to "geometrical optics" and "physical optics" when we arrive at interference and diffraction. So Hecht's first sentence in his diffraction chapter talks about "an intricate shadow made up of bright and dark regions quite unlike anything one might expect from the tenets of geometrical optics". Born and Wolf also start out with "the simple geometrical model of energy propagation was inadequate" when they begin diffraction.

I'm unsure how focusing of an electron beam through an single atom relates but I would be happy to read more about that. At least as an optical analog at lower energies this would also be refraction as the electron wave encounters a gradation in the medium. To be sure, diffraction and refraction are naming effects at sharp classical boundaries and they don't apply crisply to continuous changes.

Diffraction in crystals of say plane waves surely creates waves at angles to the original direction and one could say the the overall effect is to "bend" the wave. But is that the best word? In my mind, I bend a wire or my elbow: I have a hard time applying that as a metaphor for crystal diffraction with so many outgoing rays, but most especially since "bend" leaps out when one uses geometrical optics. Johnjbarton (talk) 23:47, 31 December 2024 (UTC)

I did not notice that the "shadow" term was already there, mea culpa. That said, it remains inappropriate.

If we go all the way back to the original electron (and x-ray) diffraction work, in neither case are there true apertures or obstacles. Electrons are "bent" by the electrostatic potential, x-rays by the charge density. With light you get diffraction due to variations in the refractive indices, e.g. opals. (I am sure that acoustic waves going through a variable medium will show similar effects, probably others as well.)

I recognize that "bending" is not a great scientific term, but I think it is adequate. How about, which is also consistent with the last paragraph of the lead:

• Diffraction is the deviation or bending of waves from straight-line propagation, due to an obstacle, an aperture or some change in the medium they are travelling through.

To a first approximation you would use for a single atom the form from Multislice

${\begin{aligned}::q_{n}({\mathbf {X} })=\exp\{-i\sigma \int \limits _{z_{n}}^{z_{n+1}}V({\mathbf {X} },z')dz'\}::\end{aligned}}$

and you can change V(r) to charge density for x-rays, and similar for other cases. This form is called the Projected potential approximation (I need to write some pages on dynamical diffraction). N.B., the Multislice page needs a little repair, some good, some bad tweaks from when my students created it for a class in 2013. Ldm1954 (talk) 00:29, 1 January 2025 (UTC)

Unfortunately we do not agree. The classical explanation of the original electron/xray diffraction work would use Huygens principle to demonstrate constructive interference due to a periodic pattern of scattering centers lit by the incoming wave. Simple point scattering centers would give the pattern, the details of the atoms is a higher order effect. The bending of sound waves in variable media and opalescence are generally considered refraction.

So let me ask you: how would you describe refraction? To me it is "bending due to change in the medium", or as Hecht says it: The fact that the incident rays are bent or "turned out of their way" as Newton put it, is called refraction. Johnjbarton (talk) 01:41, 1 January 2025 (UTC)

Sorry, but you are wrong on this, it is not a disagreement issue. The first papers used geometry, but got a slightly wrong result. Hans Bethe gave the correct explanation by solving Schroedinger's equation, with the slight glitch that he did not use the relativistic wavelength. See references in Electron diffraction#Waves, diffraction and quantum mechanics.

Your Huygens approach is an approximation of delta functions in the multislice formulation for the potential scattering; the propagator is the Greens function. A decent source is John Maxwell Cowley's book or more recently Peng, Dudarev & Whelan. Ichizuka's papers are decent as well, or the original Cowley-Moodie papers.

Refraction is quite different. For electrons it is related to the average direction and how this changes due to the mean inner potential. It comes from the boundary conditions of equal probability current. That is also in Peng, Dudarev and Whelan and a few other places. Refraction is due to a discontinuous change, and the same form applies to photons, matter waves, brownies etc -- for photons use the Poynting vector.

There are a stack of sources in Electron diffraction; I can send some papers tomorrow if needed.

N.B., the multislice approach is a numerical solution that has also been used for x-rays, neutrons, light. It is similar to Pendry's method for LEED, but for high energies back scattering is negligable. Ldm1954 (talk) 02:31, 1 January 2025 (UTC)

N.B. for refraction you match the derivatives across the boundary, for instance of a prism for light or a crystal for matter waves or xrays. More can be found in the sources cited at https://journals.jps.jp/doi/10.1143/JPSJ.18.1306 as one example, it is well documented. Ldm1954 (talk) 03:41, 1 January 2025 (UTC)