# Talk:Diffraction grating

WikiProject Physics (Rated B-class, High-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
B  This article has been rated as B-Class on the project's quality scale.
High  This article has been rated as High-importance on the project's importance scale.
WikiProject Spectroscopy (Rated B-class, Top-importance)
This article is within the scope of WikiProject Spectroscopy, a collaborative effort to improve the coverage of Spectroscopy on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
B  This article has been rated as B-Class on the project's quality scale.
Top  This article has been rated as Top-importance on the project's importance scale.

## Vinyl diffraction grating

The jupiterscope is a wibbly vinyl diffraction grating that could be placed on a camera lens There are also candies with diffraction gratings pressed onto them to have vivild pure colors These are of note as the jupiterscope could make greasy glass also become a diffraction grating from the impression The candy reminds us that cheap nanoprinting on carbohydrates is possible

## Gratuitous Science Jargon

The dimension and period of the grooves must be on the order of the wavelength in question.

Why can't we just say "The size and spacing of the grooves must be the same as the light"? What purpose does explaining something with complicated terms serve?

echelle gratings

??? No link, no article, nothing. Only someone who already knows what these echelle gratings are would benefit from this comparison.Allywilson (talk) 16:39, 18 November 2007 (UTC)

A fundamental property of gratings is that the angle of deviation of all but one of the diffracted beams depends on the wavelength of the incident light.

Lost us with "angle of deviation", "diffracted beams", "incident light". Seems like 2-3 words per sentence need links to their own article.

Therefore, a grating separates an incident polychromatic beam into its constituent wavelength components, i.e., it is dispersive

"beam"? How is anything being a beam important to the fact that it is light? Is there a single instance in this article where you could not replace the word "beam" with "light"? "Constituent wavelength components" - colors. Just say colors. Then link "colors" to an article that explains how colors work, for those who need a more precise definition.

When groove spacing...

Groove spacing finally makes an appearance, SIX or SEVEN paragraphs later! This should have been used in the first place. Now look what we have, two terms meaning the same thing, in the same article. Now we have inconsistency in addition to gratuitous jargon.

Booo science club! Hooray understanding! Viva la understandionne!

"Why can't we just say "The size and spacing of the grooves must be the same as the light"? What purpose does explaining something with complicated terms serve?"

Because it isn't correct. 'Dimension' is easy to understand. 'Period' and 'order' in the context of science are also straightforward, anyone with any interest whatsoever in diffraction gratings will understand these.

"echelle gratings"

Agreed. I'll put up an article or expansion request.

"Lost us with "angle of deviation", "diffracted beams", "incident light". Seems like 2-3 words per sentence need links to their own article."

It would take a few minutes of research for any half-intelligent person to get up to speed here. Again, if you're anything to do with science, you should know these.

""beam"? How is anything being a beam important to the fact that it is light? Is there a single instance in this article where you could not replace the word "beam" with "light"? "Constituent wavelength components" - colors. Just say colors. Then link "colors" to an article that explains how colors work, for those who need a more precise definition."

## Incorrect Diffraction Equation

According to http://scienceworld.wolfram.com/physics/GratingEquation.html and some supplier's catalogs (www.thorlabs.com) the diffraction equation should be dsin(theta_incident)+dsin(theta_reflected)=m(lambda). There should be no minus sign. I'm not sure how to edit the equations, so I'll trust that someone else will do this. --128.196.213.163 21:59, 30 October 2006 (UTC)Anon

I think it's just a problem with the sign of the angles. I tried to explain that in the article. -- Pgabolde

## Unclear relation in equation

How are groove period and groove density related? Are they inverses of one another? The article leaves this unclear, but an inverse relation is implied when the units are compared... —The preceding unsigned comment was added by MyOwnLittlWorld (talkcontribs) 16:39, 27 February 2007 (UTC).

Yes it's just the inverse. I clarified the article. -- Pgabolde

## Incorrect Example?

Rainbow-like diffraction produced by an LCD screen in direct sunlight. Note: the screen was off.

I don't think an LCD can cause a diffraction pattern, and pretty sure if it could it wouldn't look like that. Is the picture instead an example of Newton's rings, caused by the close but imperfect separation between the LCD surface and the protective plastic screen? Atropos235 19:12, 18 February 2007 (UTC)

Agreed. The color pattern in the photograph, is in no way reminiscent of the pattern created by a periodic structure (like the pixels of an lcd in this case.). Also the spacing of the pixels is too large to produce significant diffraction effects with visible light. I would guess what we are seeing has to do with the polarization of light, passing through the polarizer in the LCD screen. Transparent plastics have the property that they can rotate the polarization of light with a degree depending on how much stress is applied to the material, this effect is also wavelength dependent. This could cause different regions to reflect different colors of light. --V. 03:36, 3 March 2007 (UTC)

Since it's not clear whether the cell phone picture exhibits grating diffraction or not, I removed it for now. It looks like a thin-film effect to me. -- Pgabolde

## lines-per-inch of a CD or a DVD

What is the lines-per-inch of a CD or a DVD?-69.87.204.209 21:02, 1 June 2007 (UTC)

## Intro sentence too hard

Does anyone feel up to the challenge of writing something easier to understand? If I didn't already know what to expect I wouldn't understand it. RJFJR 17:16, 14 June 2007 (UTC)

Better now? Han-Kwang 18:30, 14 June 2007 (UTC)
Much better. Thank you. RJFJR 22:23, 14 June 2007 (UTC)

## Example of the cd/dvd

It is my understanding that it isthe thin film effect that causes interference patterns in reflected light from the dataside of cd's and dvd's and that the example on teh page needs to be removed.Allywilson (talk) 16:40, 18 November 2007 (UTC)

## Editing of the article

I have re-arranged this article, and added some new bits to make it, I hope, more comprehensible, and to have a more logical structure.

The section on 'gratings as dispersive elements' is still a bit of a mish-mash, and I will do more work on this. I don't know enough about the manufacture of gratings to do anything with this section, but I suspect it is very sketchy and imcomplete Epzcaw (talk) 13:28, 26 May 2008 (UTC)

## Diffraction efficiency

I believe that it is important to discuss the concept of diffraction efficiency either within this page or on a page of its own. I think it should be a section within this page. To those who don't know diffraction efficiency is what percent of the incident light is diffracted to a particular diffraction order in reflectance or transmitance (The efficiencies are generally different). Resonant gratings can be designed that for specific conditions (angle, wavelenth, polarization) the diffraction efficiency of a particular order will be 100%. Eranus (talk) 07:44, 24 July 2008 (UTC)

## Diffraction from a photonic crystal point of view

This is a bit more for the physics students and not the classical way to view diffraction. Since gratings are periodic, one can use the formalism of solid state physics which deals with periodic structures on the atomic scale. Anyway a homegeneous surface can not change the momentum (propagation vector, k) parralel to the surface. This is the relation between translational symmetry and conservation of momentum. A periodic structure conserves only crystal mometnum also known as quasi momentum, it means that the momentum is conserved up to a reciprocal lattice vector K (1/peiod), so if the grating is periodic in the x direction and the structure is homogeneous in the y direction, then

$\vec{k_{out}} = (k_x+mK,k_{y},\sqrt{(n\omega/c)^2-(k_{x}+mK)^2)-k_{y}^2})$

where m is the diffraction order and n the refractive index in the region of diffraction (reflection or tansmition). The way form here to the grating formula is very short, all form symmetry point of view. Will be glad to hear opinions if this should be included (Of course better worded)Eranus (talk) 07:44, 24 July 2008 (UTC)

I certainly think the description of diffraction gratings which uses QED should be included or some form of explanation using photons - it's quite important to realise that this can't just be explained using waves.

Anon. —Preceding unsigned comment added by 92.15.40.214 (talk) 18:49, 7 August 2009 (UTC)

Hi, everyone. I'm new to this. To get the ball rolling on QED in this article, I've written up a little piece using Feynman's example in his book. However, I'm young, with much to learn in the wonderful field of quantum electrodymanics, in which I've just begun to swim—nearly all of my knowledge of it comes from Feynman's book. So please, edit the heck out of this, both in language and in content—it would be nice to see a quantum electrodynamical explanation of diffraction gratings here. Also, I'll scan the images from QED when I get the time.

QED (quantum electrodynamics) offers a derivation of the properties of a diffraction grating in terms of photons as particles. In short, QED models photons as following all paths from a source to a final point, each of which has a certain probability amplitude, which can be represented as a vector or complex number (equivalently), or as Richard Feynman simply calls them in his book on QED, "arrows". For the probability that a certain event will happen, one sums the probability amplitudes for all of the possible ways in which the event can occur, and then takes the square of the length of the result. The probability amplitude of a photon from a monochromatic source, in this case, is modeled as an arrow that spins rapidly until it is 'evaluated' at its final point. (The reason for the quotes around 'evaluated' is that this spinning is actually dependent on the time at which the photon would have left the monochromatic source, as the probability amplitudes of photons do not spin while they are in transit.) So, for example, for the probability that light will reflect off of a mirror, one sets the photon's probability amplitude spinning as it leaves the source, follows it to the mirror, and then to its final point (even for paths that do not involve bouncing off of the mirror at equal angles) and then 'evaluates' it at the final point; next, one sums these arrows (in a standard vector sum), and squares the length of the result for the probability that this photon will reflect off of the mirror. (For a simplification, the arrows representing these probability amplitudes are made an egual standard length though there are, in actuality, very minor variations.) The times these paths take are what determine the angle of the probability amplitude arrow, as they 'spin' at a constant rate (which is related to the frequency of the photon). Now, the times of the paths near the classical reflection site of the mirror will be nearly the same, so as a result the probability amplitudes will point in nearly the same direction—thus, they will have a sizable sum. As we examine the paths towards the edges of the mirror, we find that the times of nearby paths are quite different from each other, and thus we wind up summing vectors that cancel out quickly (see image). So, there is a higher probability that light will follow a near-classical reflection path than a path further out. However, a diffraction grating can be made out of this mirror, by scraping away areas near the edge of the mirror that usually cancel nearby amplitudes out—but now, since the photons would not reflect from the scraped-off portions, the probability amplitude pointing, say, to the right can have a sizable sum. Thus, this would let light of the right frequency sum to a larger probability amplitude (which, of course, then has its length squared for the probability that light will reflect from the selected region). This description of course involves many simplifications: a point source, a "surface" that light can reflect off of (thus neglecting the interactions with electrons) and so forth. However, this approximation is a reasonable one to illustrate a diffraction grating conceptually. Light of a different frequency can also use the same diffraction grating, but with a different final point.[1] Trmwiki (talk) 21:58, 2 October 2011 (UTC)

Since none have objected so far, I'll go ahead and post this in the main article. Feel free to change anything, of course. Anyway. Thanks! — Preceding unsigned comment added by Trmwiki (talkcontribs) 22:00, 2 October 2011 (UTC)

## Diffraction Gratings tutorial by J.M. Lerner and A. Thevenon

The tutorial is divided in the following section: Section 1: DIFFRACTION GRATINGS ? RULED & HOLOGRAPHIC Section 2: MONOCHROMATORS & SPECTROGRAPHS Section 3: SPECTROMETER THROUGHPUT & ETENDUE Section 4: OPTICAL SIGNAL?TO?NOISE RATIO AND STRAY LIGHT Section 5: THE RELATIONSHIP BETWEEN WAVELENGTH AND PIXEL POSITION ON AN ARRAY Section 6: ENTRANCE OPTICS

It is a tutorial, by J.M. Lerner and A. Thevenon, part of the HORIBA Jobin Yvon company website, on the optics of spectroscopy. It covers: diffraction gratings - ruled and holographic; monochromators and spectrographs; spectrometer throughput and etendue; optical signal-to-noise ratio and stray light; the relationship between wavelength and pixel position of an array; and entrance optics.

This article will be very helpful on the diffraction grating page as an external link (http://www.jobinyvon.com/SiteResources/Data/Templates/1divisional.asp?DocID=616&v1ID=&lang=) .Afrine (talk) 14:47, 21 November 2008 (UTC)

## In nature

"Diffraction gratings are also present in nature. For example, the iridescent colors of peacock feathers, mother-of-pearl, butterfly wings, and some other insects are caused by very fine regular structures that diffract light, splitting it into its component colors." Does this also apply to other iridescent surfaces found in nature, like the leaves of some plants ? If not then what is the iridescence mechanism ? 77.100.112.34 (talk) 16:42, 10 September 2010 (UTC)

The mechanism of iridescence is typically interference, whereas the diffraction grating reflects light of different color at different angles. Iridescence usually is caused not by regular structures that reflect light, but by placing two or more transparent surfaces parallel to each other, separated by a distance equal to some multiple of a wavelength of light. Iridescence is caused by the same thing which allows a Fabry-Perot interferometer to select certain colors by constructive interference, while eliminating other colors by destructive interference. The same effect allows a laser to work.
As an example, a thin sheen of gasoline on top of a puddle of water does not produce fine lines, but rather produces a very thin layer with almost parallel refective surfaces, which causes interference. The thickness of the layer determines what color will be reflected back to you. Slight variations in thickness produce the different rainbow colors, which move and shift as the liquid flows. Light passing through at an angle "sees" the distance between layers as being thicker than light passing through at a perfect 90 degrees, so constructive interference will boost the colors of longer wavelengths as the viewing angle is increased. I hope that helps answer your question. Zaereth (talk) 17:46, 10 September 2010 (UTC)
I did a little research here. The book Nature's palette: the science of plant color By David Webster Lee, page 255, says, "Structural coloration is common in animals, particularly in insects and birds, as in the peacock feather commented on by Robert Hooke. Such colors are almost always caused by thin-film interference. The iridescent green wing chevrons of the Rajah Brooke birdwing butterfly we encountered in Malaysian rainforests are produced in this way, by multilayered structures in the wingscales that interfere with visible light." He goes on to describe exactly how this happens, much like I've described above. Mother of pearl is definitely not iridescent from the effects produced by a diffraction grating> The only natural phenomenon I found where such regular structures exist to produce the very angle specific diffracted reflection on a scale large enough to be visible to us is a rainbow.
Perhaps the unsupported statements from the article should be removed, because the sources so far contradict what it says. Zaereth (talk) 01:47, 11 September 2010 (UTC)
I have made some changes the article to match the sources which I've found. If there are any questions or comments, please leave them here. I left out the stuff about a rainbow, however. Looking for a better source I found this site, http://physics.bu.edu/py106/notes/Diffraction.html . A rainbow is caused by dispersion, not diffraction. My bad. That site gives a good description of diffraction, and may be a good reference for the article. Zaereth (talk) 22:48, 14 September 2010 (UTC)
The following posted here originally: http://en.wikipedia.org/w/index.php?title=Diffraction_grating&oldid=398524127 "I believe that butterfly wing color is due to regular diffraction patterns in their scales. See "Butterfly" : Butterflies are characterized by their scale-covered wings. The coloration of butterfly wings is created by minute scales. These scales are pigmented with melanins that give them blacks and browns, but blues, greens, reds and iridescence are usually created not by pigments but the microstructure of the scales. This structural coloration is the result of coherent scattering of light by the photonic crystal nature of the scales.[5][6][7]" I have no idea on the accuracy of this claim, but moved it to Discussion as it is more accurate here for the time being. Zklink (talk) 05:18, 7 December 2010 (UTC)
The structures in the butterfly scales, at least as far as I have read, are thin-film structures (photonic crystals) and not so much diffraction. The easiest way to tell is to ask the question, does the iridescence produce the entire rainbow of colors as the viewing angle changes, or is the color change limited to just a few colors? Diffraction will spread out the entire spectrum, similar to the way a prism will. With thin-film interference, the constructive interference usually causes a change from one color to one other, (ie: from green to blue). The destructive interference eliminates the rest of the spectrum. The book Dynamic fields and waves By Andrew Norton gives a pretty good description of how thin-fim interference works in butterflies. On page 111, it shows an illustration of the structures in the butterfliy scales that causes such interference. (I'm not saying that there aren't cases where diffraction does play a role, but I haven't found them yet.) Zaereth (talk) 17:18, 7 December 2010 (UTC)

On the NASA astronomy picture of the day website, yesterday, was posted a photo of iridescent clouds, which are apparently caused by the diffraction grating phenomenon. I don't have any reliably sourced info on this, but if someone does, it may be worth adding to the article. Zaereth (talk) 20:44, 9 February 2011 (UTC)

## Diagram urgently needed

Can anyone find a diagram to illustrate the grating equation explanation in the "Theory of Operation" section? In particular it should show the wavefronts and/or interference pattern in relation to the angle theta_i, which is hard to visualize from the text. (As well as d,lambda of course, and a value for m.) 84.227.237.33 (talk) 18:32, 3 April 2014 (UTC)

1. ^ Feynman, Richard (1985). QED: The Strange Theory of Light and Matter. Princeton, New Jersey: Princeton University Press.