# Talk:Diffraction

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## 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 (talkcontribs) 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,

${\displaystyle 2\pi d\sin \theta =\lambda .}$

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)

## 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 (talkcontribs) 07:04, 8 September 2015 (UTC)

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