# Talk:Prism

## Bit Rot

This page is in serious need of work. Some mystical rubbish has crept in here about refraction of Dark.

I don't like to flag work needing attention, but I'm battling on other fronts right now. — Xiongtalk* 23:29, 2005 August 7 (UTC)

The caption to the first image doesn't make sense to me — what does such that the colored edges meet mean? I will change this unless someone can explain. Michael Fourman 07:18, 6 March 2007 (UTC)

I don't have time to think about it right now, but it sounds similar to some of the things discussed in the Theory of Colours article. The gist of that is that the familiar spectrum we think of coming out of a prism only appears if the beam of white light is narrow. If the beam is wider, one sees a white beam with coloured edges come out of the prism, not a traditional spectrum. Goethe (the author of Theory of Colours) developed a theory of colour based on this. While Newton's work best addresses the physical properties of light, Goethe's model perhaps better deals with the peculiarities of colour perception in human beings.--Srleffler 08:11, March 6, 2007 (EST)

## Shape of Prism

Note: "Traditionally, these prisms are built with a triangular base and rectangular sides." Shouldn't this be rectangular base with triangular sides? -- 217.129.246.156

Sorry, no. The object you describe is a pyramid.
In geometry, a prism is any solid generated by extrusion of a polyhedron. The classic optical prism is generated by extruding a triangle, which is its base. It is a right prism, so its sides are rectangles. — Xiongtalk* 02:05, 2005 August 26 (UTC)

What are the shortest and longest spectrum wavelengths capable of being dispersed by a prism?

Any wavelengths that can be transmitted through the material of the prism can be dispersed. Typical optical glass transmits from somewhere in the near ultra-violet up to a few microns wavelength (near infra-red). Other optical materials may transmit a wider range of wavelengths.--Srleffler 12:17, 23 February 2006 (UTC)

## DSOM

I removed the following from the article, but perhaps we should have a discussion about it:

Prisms in popular culture

This keeps popping up in the article. The fact that someone used a prism on an album cover once is just not that important a fact about prisms, even if the album does happen to be especially famous. It is barely justifiable to even link to DSOM from here. Since the person who added the above felt it was confusing having a link in the See also section, I removed the DSOM reference from the article altogether.--Srleffler 02:33, 27 September 2006 (UTC)

## what are prisms made from?

I came here to find out what prisms are made from, and I still don't know. glass? crystals? —Preceding unsigned comment added by Raw melk (talkcontribs)

It says "the glass of the prism" in the 2nd paragraph. But any transparent material will do. Han-Kwang 22:14, 23 November 2006 (UTC)
Yes, typically glass, but sometimes prisms can be made out of plastic instead. Prisms made out of crystals are also pretty common; especially for polarizing prisms.--Srleffler 05:41, 24 November 2006 (UTC)

## Confusion

In the beginning of the article, it is said that "In optics, a prism is a device used to refract light, reflect it or break it up (to disperse it) into its constituent spectral colours". From this I would suppose that there are refractive, reflective and dispersive prisms. But later in the article, prisms are classifed as reflective, dispersive or polarizing. Could we make this more clear? Jorge Peixoto 10:58, 29 November 2006 (UTC)

Fixed. Han-Kwang 11:20, 29 November 2006 (UTC)

## Prism definition

Srleffler reverted two changes that I made:

• with flat polished surfaces that are not parallel to each other. - What is wrong with this? Anything that classifies as an optical prism should have at least one pair of flat optical surfaces that are nonparallel. Otherwise it is simply called a substrate, optical window, or rod. From the Melles Griot catalog: "...blocks of optical material with flat, polished sides arranged at precisely controlled angles..."
• If not indicated otherwise, the term prism usually refers to a triangular prism - My main argument is that about half of the articles that linked to prism (optics) use wordings similar to "the light was sent through a prism" and were actually referring to triangular prisms to disperse light. (I changed most of them to direct to triangular prism (optics)). Although in optics catalogs such as those from Melles Griot, Newport, and Thorlabs, prisms are always specified as "right-angle prism", "equilateral dispersing prism", "penta prism", it seems that in colloquial usage the word "prism" most often refers to a triangular dispersing prism. A google search on "prism light" confirms this. I think the article should respect this common usage. Maybe the wording "usually refers to" was too strong; how about "in colloquial usage, the term 'prism' most often refers to a triangular dispersing prism"?—Preceding unsigned comment added by Hankwang (talkcontribs) Nov. 30 2006
• My main reason for the change to the first sentence was that it failed to specify that a prism is an optical device, and failed to give any indication of its purpose. Instead, it said that a prism is a "transparent object...with flat polished surfaces that are not parallel to each other". Function is more important than form: A prism is an optical device that refracts light at flat optical surfaces. Also, in general some of the surfaces can be parallel to other surfaces. Your version implied that this was not the case. I understand your concern though with distinguishing it from a window or flat. We can probably come up with wording that addresses both of our concerns.
• Yes, "usually refers to" was too strong, and isn't true within the field of optics. I'm fine with a statement about common colloquial or historical usage.--Srleffler 15:31, 30 November 2006 (UTC)

## Newton

There have been several attempts to rephrase the paragraph on Newton, which I believe change the intended meaning of the paragraph. If I am right, there are a couple of sentences that are poorly phrased, which are leading to confusion. I would rephrase them, but need confirmation that my interpretation is correct. The section in question reads:

Isaac Newton first thought that prisms split colors out of colorless light. Newton placed a second prism such that a separated color would pass through it and found the color unchanged.[citation needed] He concluded that prisms separate colors.

I believe what is intended here, is that Newton at first thought that prisms produce colors in some fashion from light that is intrinsically colorless (white). By performing the experiment described, he convinced himself that prisms separate preexisting colors, and therefore that white light is not colorless, but rather is composed of a mixture of colors. If I am right, the paragraph needs to be rephrased, because other editors seem to routinely miss the fact that a change in Newton's point of view is seen here. What is needed is a confirmation of this statement from an external source, so we can verify what meaning was intended.--Srleffler 00:31, 15 March 2007 (UTC)

Sorry, but this is my fault. Quite some time back, it was I that added the word 'colourless' in the place of 'white' light. I did this because today it is often assumed that colourless light is white. Someone subsequently didn't see the sense of it, and rewrote the sentence by adding the phrase, 'first thought' -- and this changed the meaning enough that it devolved into the subtle shift you noted above. There is no change in Newton's thought here from colourless to white. He assumed it was white all along, and used the experiment crucis to prove his point. Johnrpenner 16:43, 7 July 2007 (UTC)

## Explanation first, then history

I have moved the "history" section back after the "how prisms work" section. It is often better in science articles to explain the substance of a topic before the history of the subject. How things work is fundamental in a science article. The history of how something was discovered is of secondary importance. Additionally, the text in the "history" section is not really a history of prisms, but a much narrower bit of history about how the spectrum came to be understood using prisms. --Srleffler (talk) 05:27, 26 April 2008 (UTC)

nice edit. now we're two steps closer to the great overhaul this article needs. Johnrpenner (talk) 15:17, 26 April 2008 (UTC)

i still don't get how to refract the light from a prism, it didn't explain theat whatsoever, do you shine it through the face, a corner? please could someone clarify? —Preceding unsigned comment added by 173.33.18.41 (talk) 22:47, 18 November 2008 (UTC)

Where did this history section go you speak of?! Someone please bring it back, I'm having a hard time finding anything online referencing the origins of the optical prism. -Luminaux (talk) 02:41, 5 March 2010 (UTC)

It's still there. The section was retitled Prisms and the nature of light. It isn't really what you're looking for. --Srleffler (talk) 05:25, 5 March 2010 (UTC)

Just how old are prisms, anyway? Newton used them, sure, but what's the oldest recorded instance of a prism? How long have they been around? Alphapeta (talk) 07:11, 6 February 2017 (UTC)

## Deviation angle

I'm not sure the new section on deviation angle should stay as it is. The form given is only a good approximation if the wedge angle of the prism is small. It's great for a thin wedge, but I'm not sure if it's accurate enough to use for a more typical dispersing prism. Perhaps it would be better to not try to derive a formula, but just give the accurate expression without derivation.--Srleffler (talk) 04:24, 24 June 2010 (UTC)

I unintentionally undid Srleffler's edit here, which was removing a link to dispersion. This happened because I was making a major edit to the same page and the merge editor couldn't parse the differences easily because there were so many. I still think the link is a good idea, since dispersion is not explained on this page. Note that there are two types of dispersion here, and they're not the same thing: spectral dispersion (dependence of deviation angle on wavelength) and glass dispersion (dependence of refractive index on wavelength). This may confuse some people, and so I think a link is a good way of helping people out.NathanHagen (talk) 02:04, 3 November 2011 (UTC)

I understand about the edit conflict, but disagree about the value of the link. Even if the page needed a second link to the article on dispersion, the place where you put it is not the right place. You linked dispersion at the end of a section that has dispersion in its section heading, with two previous uses of the term in that section alone. If the article needed a second link to the article, it would have to go at the beginning of that section.
There is no difference between "spectral dispersion" and "glass dispersion". They are the same thing. The refractive index varies with wavelength. By Snell's law, this causes angle of refraction to vary with wavelength. --Srleffler (talk) 02:28, 3 November 2011 (UTC)
I removed the confusion by removing the confusing use of "dispersion" to refer to variation in angle of refraction with wavelength.--Srleffler (talk) 02:43, 3 November 2011 (UTC)

## Dispersion image

MaterialScientist, why did you delete the animated image of wave dispersion in a prism? Your edit summary describes the image as misleading. How so?--Srleffler (talk) 06:48, 8 February 2011 (UTC)

I believe the top image it is an illustration "dumbed down too much", as it shows optical waves flowing through air with wrong speed and size. They are also mysteriously coherent. The image under is uncomparably better to my eye - it is more "real", and correctly shows all beams (reflection too). Why using incorrect simulation when the real prism action is illustrative enough? Materialscientist (talk) 07:03, 8 February 2011 (UTC)
The lower second illustration worries me a bit. It appears to show all of the dispersion happening at the exit face of the prism, which is wrong. I'm not sure if this is a rendering error, or if the angular spread inside the prism is just too small to see. Besides illustrating the important relationship between wave speed in the medium and dispersion, the animated image illustrates the important point that the dispersion happens inside the prism, not when the waves exit. Illustrations of prism dispersion so often fail to illustrate this correctly.--Srleffler (talk) 01:33, 9 February 2011 (UTC)
The middle image is also a simulation, as obvious upon magnification and from absence of reflection from the 2nd interface. The bottom (added) is real, unfortunately with an unpolished side faces. I guess we often don't see colors inside the prism because of insufficient dispersion (within the beam divergence) and because of extra diffusion (color mixup) due to scattering inside the prism (we see the inside beam only because of scattering on glass inhomogeneities). I'll revert myself. Materialscientist (talk) 05:25, 9 February 2011 (UTC)

I like the animated schematic illustration. It shows how the different colors correspond to different wavelengths, and different speeds inside the prism, and how these relate to the different angles. The other one is good, too, but doesn't connect to wavelength and speed at all. Dicklyon (talk) 07:12, 8 February 2011 (UTC)

Well, if illustrating the propagation speed is essential then I might be wrong. Materialscientist (talk) 07:33, 8 February 2011 (UTC)
I think the animation is very good and should be kept. Dauto (talk) 17:40, 8 February 2011 (UTC)
I think it should be kept per User:Dicklyon. The relative wavelengths, and speeds might even be more intelligible if the image was larger (just a thought). ---- Steve Quinn (talk) 22:28, 8 February 2011 (UTC)
The new image in the lead of the Optics article does not serve the article as well as either of the other two in this page. In my (humble) opinion, this image needs to be removed. (Sorry). I prefer image that was there in the first place.---- Steve Quinn (talk) 06:32, 9 February 2011 (UTC)

The previous image (middle here) was misleading in being neat and looking like real photo, yet showing no dispersion inside the prism and no reflection on its second face. I've boldly replaced with the bottom one, which is real, but ugly. I would only welcome if someone finds a better lead image. Materialscientist (talk) 06:58, 9 February 2011 (UTC)

### Inconsistency in Illustrations

[Moved to end by srleffler. Poster presumably did not see the discussion above.]

is it only me, or does anyone else think it is inconsistent for the two prism illustrations to show two different things within the same article?

the first illustration clearly shows a white beam of light entering the prism, and It Stays White inside the prism, and you see the colours fanning out -- only after the second refraction -- where it EXITS the prism.

the second illustration shows a white beam of light entering the prism -- where you see the colours begin to fan-out WITHIN the prism, and then they exit the prism even further fanned-out from the second refraction.

they can't both be right -- so which is it!? are the colours fanned out when they ENTER the prism, or when they EXIT the prism? -- and who is going to remove the incorrect illustration, or make sure that both illustrations show the same thing? Johnrpenner (talk) 01:22, 26 April 2011 (UTC)

The first illustration is flawed, although the actual best way to illustrate this is not so clear. I removed the flawed image, and added a photograph further down in the article.
Light definitely begins to fan out as soon as it enters the prism. Different colours of light refract at different angles at the entrance face, and again at the exit face. One doesn't normally see the individual colours within the prism for several reasons. First, as noted above, the side faces of the prism are often not transparent. Even if they are transparent, the scattering of light inside the prism is weak and may be overwhelmed by multiple reflections of beams off of the surfaces. More fundamentally, though, the colours only become apparent when the beam has spread enough that they no longer overlap. The beam has to be extremely narrow to see colours inside the prism. (See Theory of Colours for more on what happens when you put wide beams of light through dispersive prisms.)
Because of the complexity of what actually happens inside the prism, I prefer the animation already present in the article, which doesn't attempt to show how the light in the prism looks, but rather shows schematically what is happening. If we want to see what the phenomenon looks like, we are better off with a photograph, which shows us how it actually looks rather than how someone thinks it ought to look.--Srleffler (talk) 02:13, 26 April 2011 (UTC)

## Merge with Dispersive prism

See Talk:Dispersive prism#Merge. --Hans Dunkelberg (talk) 10:54, 3 July 2011 (UTC)

## Need a source for Newtons prism experiment influencing Locke and metaphysics

Especially since Neither of those articles mention Newtons prism experiments. - Rod57 (talk) 23:36, 8 April 2015 (UTC)