# Why are diffraction gratings not called interference gratings? [closed]

It seems to me that diffraction gratings are completely described by the double slit experiment-why then is it called a diffraction grating?

-

## closed as unclear what you're asking by ACuriousMind, Gert, MAFIA36790, John Rennie, Kyle KanosNov 17 '15 at 11:17

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

In my experience, these terms are loose, and sometimes used for historical reasons. "Interference" may relate to the general idea of several waves overlapping. "Diffraction" is most often used when an object of a finite size is put in the path of the wave. For "diffraction grating", it might be due to the fact that each groove is such an object. But it could also have been called "interference grating". – fffred Aug 15 '13 at 17:41
Diffraction is a subset of interference. For example, say you have a standing wave on a string, with a node in the middle. The node exists because of interference between two traveling waves. There is no diffraction going on here. – Ben Crowell Aug 15 '13 at 22:01

It seems to me that diffraction gratings are completely described by the double slit experiment

I don't think this is true, unless I'm incorrectly understanding what you mean by "are completely described by." I also don't understand what the double slit has to do with the rest of the question.

Why are diffraction gratings not called interference gratings?

Interference is the mechanism that gives rise to the phenomenon of diffraction. Either name could have been used.

-
Well, comparing the double slit to the single slit experiment, it would seem to me that a diffraction grating it more akin to the double slit experiment. And the double slit experiment doesn't talk about diffraction. – user24082 Aug 15 '13 at 15:06
"And the double slit experiment doesn't talk about diffraction" - see for example en.wikipedia.org/wiki/… – John Rennie Aug 15 '13 at 16:18
I don't mean to be argumentative with this statement, but I've always thought of diffraction being the mechanism that gives rise to interference because you can have a diffraction pattern from a point source whereas interference requires a minimum of two sources. I find your remark to be interesting, I'm not sure which is 'correct'. – Joe Aug 15 '13 at 18:38
@Joe: you can have a diffraction pattern from a point source Huh? E.g., if I throw a single rock in a pond, I don't see a diffraction pattern...? – Ben Crowell Aug 15 '13 at 20:04
@BenCrowell If you have a plane wave incident upon an infinitely small pinhole you can image its diffraction pattern. Can you call that an interference pattern if there is only one source? Maybe? Depends who you ask I suppose – Joe Aug 15 '13 at 21:17

I asked myself the exact same question. This PDF file from Harvard says that indeed diffraction gratings should be instead called interference gratings (page 13, first paragraph of "Remarks").

EDIT : Here is the paragraph from the link, as suggested by @Kyle Kanos :

A diffraction grating should more appropriately be called an “interference grating,” because it is simply an example of N -slit interference. It is not an example of diffraction, which we will define and discuss in Section 9.3.1. e’ll see there that a feature of a diffraction pattern is that there are no tall side peaks, whereas these tall side peaks are the whole point of an “interference grating.” However, we’ll still use the term “diffraction grating” here, since this is the generally accepted terminology.

-
You should be able to quote the relevant passage from Morin's book draft (you can use > text to get a block quote) to show why he thinks so, otherwise this might be categorized as a 'link-only answer' and be deleted. – Kyle Kanos Nov 13 '15 at 21:05
Passage added, thanks. – user1901473 Nov 16 '15 at 20:12

They are two different phenomena.

Diffraction is what happens when waves encounter an obstacle, describing the way waves are distorted by the obstacle.

Interference is what happens when two sets of waves overlap.

It just so happens that in two slit diffraction, a wave becomes two sets of waves on the other side of the diffraction grating, allowing interference to occur. However the grating is not causing any interference directly - it is simply causing a single wave to become two waves which can then overlap. (it would be trivial but possible to keep the two waves separate on the far side of the grating so that diffraction occurred without interference)

-
Interference is what happens when two sets of waves overlap. Not just two. It can be any number. Diffraction is what happens when waves encounter an obstacle, describing the way waves are distorted by the obstacle. And the physical explanation of this is Huygens' principle and interference of the waves described by Huygens' principle. So these are not two different phenomena. Diffraction phenomena are a subset of interference phenomena. – Ben Crowell Aug 15 '13 at 20:51
You are right to bring up the Huygens–Fresnel Principle - I didn't know about that, so I now see I didn't know enough to understand fully where the question might have been coming from. However from what I have read about it, it is a "principle" rather than a true description of what happens - i.e. an idealised analogy that provides a useful heuristic explanation, rather than necessarily what physically happens, for which reason it has been described as providing "the right answer for the wrong reason". Would you agree with that summary? – Gwilymino Aug 16 '13 at 20:29
The WP article you're quoting links to this page, which gives a very complete discussion: mathpages.com/home/kmath242/kmath242.htm – Ben Crowell Aug 16 '13 at 21:17
From the 4th paragraph of your link: "... there is a wide range of opinion as to its scientific merit. Many people regard it as a truly inspired insight, and a fore-runner of modern quantum electro-dynamics, whereas others dismiss it as nothing more than a naive guess that sometimes happens to work." i.e. the idea of explaining diffraction as being composed of myriad interference patterns is an analogy that roughly fits rather than a scientific and certain description of what is actually happening. Regarding the original question, wouldn't that be why gratings are named the way they are? – Gwilymino Aug 17 '13 at 7:54