Talk:Specular reflection

Old comments

I've removed the material on computer graphics and holography as not all that relevant. Computer graphics seeks to model the real world, so of course it includes specular reflection. Having a good model for specular reflection is obviously very important for computer graphics. Computer graphics is not, on the other hand, that relevant to specular reflection. The article could say something about how specular reflection is modeled in computer graphics, but the section I deleted did not do this. It didn't really add any useful information to the article, but rather focused on showing off the pretty picture the contributor made.

I moved the holography section to talk:holography, for similar reasons. It asserted that specular reflection is important for that type of holography, but did not deal with why holography is important or relevant to specular reflection. --Srleffler 04:33, 6 September 2006 (UTC)

It is too bad that an article with a title like Specular reflection cannot be seen as a good place to discuss why and how this technique is used in computer graphics.

Rather than fight over and over by removing other peoples work, and making derogatory comments about their efforts, I will place information about mirrors, reflection and other rendering techniques in other articles, where the article name may not be appropriate, but at least they will be available for those researchers to whom they are important. ADH 05:41, 6 September 2006 (UTC)

Spelling

The last bit of this page contains a lot different spellings of the same thing.... now some are clearly just typos, but the text implies that the author is seeking to differentiate between the different spellings so I'm an not sure whether to clean it up or not (I'm not a physical optics person so I'd rather check first!). For the record, in the last paragraph, there was: "specilar reflection" "specular refleciotn" "specuilar reflection" "electomagentic" "basiacally" "esist" "specular reflectiin" "otder". 86.53.48.59 14:11, 15 April 2007 (UTC)

Unclear information

The article, in it's current form, does not adequately explain in layman's terms, how specular reflection works in quantum mechanics. I was hoping to find that information on this page. i kan reed 16:33, 14 November 2007 (UTC)

The phrase "This is in contrast to diffuse reflection, where incoming light is reflected in a broad range of directions" is unclear to me. What does "a broad range of directions" mean. One photon can only travel in one direction at a time. So does this mean that, unlike specularity where we can predict the angle of reflection, we can not predict the reflected angle of a diffused reflection? GreenEconomy (talk) 02:05, 2 January 2009 (UTC)

Yes, if all you have is a macroscopic description of the surfaces, you can predict the angle of reflection from a specular surface but not from a diffusing one. A diffuse surface is microscopically rough. A narrow beam of light striking the surface is reflected in a broad range of directions, with the exact angular distribution depending on the surface. If you reflected a single photon off of the surface, I believe you would find that its angle of reflection is random, with probability distribution matching the intensity distribution for the reflection of a beam of light.--Srleffler (talk) 05:26, 2 January 2009 (UTC)

How much light not specularly reflected?

Is there a first-order approximation of how much light is not reflected by a perfectly glossy dielectric material as a function of angle of incidence? —Ben FrantzDale (talk) 17:55, 29 April 2008 (UTC)

Yes, see Fresnel equations. --Srleffler (talk) 20:33, 29 April 2008 (UTC)

Thanks. —Ben FrantzDale (talk) 01:20, 30 April 2008 (UTC)

is a reflected photon absorbed and re emitted, or bounced like a billiard ball

apologies if this does not make sense from a QD point of view, but I did not think the article explicitly stated what happens. —Preceding unsigned comment added by 65.220.64.105 (talk) 14:20, 29 October 2008 (UTC)

Yes.

Really. The photon is governed by quantum mechanics. It's not really meaningful to ask whether it is absorbed and re-emitted, or bounced. It does both/neither/something else altogether.--Srleffler (talk) 16:39, 29 October 2008 (UTC)

Reflection (physics)

Are these two the same? Should we merge? If not, should the Reflection disambiguation page have a link here? DJ Clayworth (talk) 18:45, 27 May 2009 (UTC)

They aren't the same. Specular reflection is one specific type of reflection. Diffuse reflection is the other common type. Neither needs a link from the dab page, because the link to Reflection (physics) covers the topic, and provides links to the more specific articles.--Srleffler (talk) 03:29, 28 May 2009 (UTC)

Citations

It is posted in the article that additional citations are needed. Would video material (for example, a minute-second designator on a YouTube Video of an MIT OpenCourseware recorded class lecture) count? Raryel (talk) 19:49, 2 June 2009 (UTC)

There is nothing wrong with video material per se. Note, though, that Wikipedia requires reliable sources—generally material published by independent third parties with a reputation for fact-checking. A recorded MIT lecture might or might not qualify. Why don't you post a link here and we can take a look at it?

That said, of course for a non-controversial fact a not quite reliable source may be better than no source. --Srleffler (talk) 00:03, 3 June 2009 (UTC)

Formula

I tried using the formula in the article and I was getting weird, inaccurate results. Could someone add a proof to this page? --Humanist Geek (talk) 05:02, 16 May 2010 (UTC)

I found the flaw. The entire equation needs to be multiplied by -1. --Humanist Geek (talk) 05:27, 16 May 2010 (UTC)

Are you sure you aren't using a different sign convention somewhere in your calculation?--Srleffler (talk) 15:56, 16 May 2010 (UTC)

How can I tell if I'm using a different sign convention if the "proper" sign convention isn't even defined in the article? This is one of the reasons why I want a proof, or at least an example that shows how to use the equation correctly. But the latter is frowned upon as being unencyclopedic, whereas the former would make an excellent and welcome addition to the article.--Humanist Geek (talk) 22:43, 16 May 2010 (UTC)

The article should define and explain any sign conventions that are being used; admittedly failure to do so is common in Wikipedia physics articles. A proof is beyond the scope of the article, but should be available in the references the article cites.

Which formula is it that you are having trouble with? There are several in the article.--Srleffler (talk) 01:33, 17 May 2010 (UTC)

The formula $d_{s}=2({\hat {d}}_{n}.{\hat {d}}_{i}){\hat {d}}_{n}-{\hat {d}}_{i}$ is the one that's incorrect by "usual convention" - the terms either side of the minus are the wrong way around. In the usual convention the "incident" vector is pointing towards the surface, and the "normal" vector directly away from it. Hence the dot product $({\hat {d}}_{n}.{\hat {d}}_{i})$ term ends up negative because in reality it's equal to $cos(180-\theta _{i})$. See e.g. http://www.flipcode.com/archives/reflection_transmission.pdf . Raybellis (talk) 21:39, 25 August 2010 (UTC)

I don't think the assumed orientation of the normal vector matters; its "sign" cancels. Changing the sign of only the dot product term is clearly wrong. It's the whole right hand side of the equation that may need to be multiplied by -1. I have no objection as long as whoever makes the change explains in the article what sign convention they are using, provides a reference, and fixes all the math in that section so the sign convention remains consistent.--Srleffler (talk) 03:12, 26 August 2010 (UTC)

I didn't mean to imply that only the dot product sign is incorrect. I know how to draw the correct geometrical derivation of the formula, if someone could recommend a program that'll produce SVG output. Raybellis (talk) 15:17, 26 August 2010 (UTC)

The formula is wrong; it has nothing to do with any sign convention. The correct formula is: $d_{s}={\hat {d}}_{i}-2({\hat {d}}_{n}\cdot {\hat {d}}_{i}){\hat {d}}_{n}$. This can be easily seen by considering the limit where the angle of incidence is close to 90 degrees. In that case the dot product approaches zero and we find $d_{s}={\hat {d}}_{i}$, which means the reflected ray moves in the same direction as the incident ray, which is correct (I've written a program that does ray-tracing and have used this formula many, many times). —Preceding unsigned comment added by Erik Lascaris (talk • contribs) 23:43, 13 October 2010 (UTC)

Whether ${\hat {d}}_{s}={\hat {d}}_{i}$ or ${\hat {d}}_{s}=-{\hat {d}}_{i}$ in the case you describe is indeed determined by a sign convention. While I agree with you that the convention you are using makes more sense, the article cites two reliable sources for the material presented. Reference 3 is available on Google books, and does present the formula in the form given in this article, albeit with notational differences. See eq'n 10.97 in that reference. If you want to change the sign convention in the article, you need to present at least one reliable source that uses the convention you prefer, and you have to describe the new sign convention in the article and fix all the formulas so that they use the same convention. --Srleffler (talk) 17:35, 14 October 2010 (UTC)

The sign convention does matter and the given formula is correct if the incident direction is defined as going from the surface rather than against it (as it is in the third reference). Although why you would choose to define the direction of the incident light like this is a mystery to me. --Kri (talk) 18:15, 14 June 2011 (UTC)

Formulas without a clear scope

It is not at all clear which might be the scope of the paragraph "calculation", and the utility of its formulas. Without an explanation in words, the formulas apparently use a complicated formalism to tell about nothing. And the title, too, is inappropriate. It perhaps might be changed in "Translation of reflection laws to matrix formalism for using them in ...". Also, perhaps it is not necessary to specify in detail to the users of such formalisms which are the tricks to get the norm of a vector.

I suggest to eliminate completely the paragraph--GianniG46 (talk) 14:04, 14 June 2010 (UTC)

I don't think the section should be deleted. It does not appear to me that the scope of these equations is limited. Rather, they give a general method for determining the reflection direction, given the incident and normal directions. The formalism is elementary vector algebra: senior year of high school, or first-year university math. It may look complicated, but if you are working with beams and flat surfaces in 3D rather than working in 2D in the plane of incidence, it doesn't get much simpler than this. Perhaps it could be titled or explained better.

I deleted the part about how to deal with unnormalized vectors. While it's a useful tip, it seemed to me to be beyond the scope of this article.--Srleffler (talk) 17:16, 14 June 2010 (UTC)

By the way, the heading you propose above is not accurate. The section shows how to calculate the specular reflection direction, using two different formalisms: vector and matrix. --Srleffler (talk) 17:21, 14 June 2010 (UTC)

OK, now it's much better. The title I was proposing of course was in some degree a joke, just to stress the need to explain which was the matter.--GianniG46 (talk) 18:06, 14 June 2010 (UTC)