Talk:Opacity

Opacity formulae
The formula for the brem. opacity may be fine for a hydrogen plasma, however needs to be different for a general plasma - dependent on Z squared. Equation (31) on pg. 58 of the NRL Plasma Formulary might be better? —Preceding unsigned comment added by 155.198.208.38 (talk) 14:17, 15 May 2011 (UTC)
 * Sounds fine to me. I think you should go ahead and change it and add the new footnote. See WP:T or ask me if you're confused about how to edit the article. :-) --Steve (talk) 18:39, 15 May 2011 (UTC)
 * All-in 66.38.29.124 (talk) 21:35, 25 June 2024 (UTC)

Moved text
I moved the contribution from 'Dinesh Pathak Ph D student GND University Asr' into the main body of the text as a new 4th paragraph and cleaned up some of the spelling. Mattgrommes 21:30, 3 August 2006 (UTC)

Molar (decadic) absorption coefficient
IUPAC recommends that &epsilon; be called the molar (decadic) absorption coefficient. See, , and especially. — DIV (128.250.204.118 08:20, 7 September 2007 (UTC))

Definition (renamed from Numerical definition)
I have made a somewhat feeble attempt to clarify and tighten up the lead paragraph; I also changed the section title, "Numerical definition", to just "Definition".

There remains some confusion about the consistency of the units and the terminology, which I need to refer to my textbooks to straighten up (is it per meter, or per gm/cm^2, etc?) Someone more expert about the terminology and the whole field of radiative transfer is really needed to do this better. Improved co-ordination between this page and the article on radiative transfer, within the general subject of which opacity is just one important concept, also needs to be addressed.

The actual calculation and tabulation of opacities, generally of plasmas and stellar atmospheres, is a major field of endeavor in itself in astrophysics (& I suppose in many other fields), and may deserve separate treatment, or a section here with references to the literature.

As it is the article seems fragmented and disorganized, though I see many good chunks. Needs TLC from someone, I think. Wwheaton (talk) 17:59, 19 February 2008 (UTC)

merge with Absorption (electromagnetic radiation)
I would suggest no merge, Opacity (optics) is a subset or narrower field than Absorption (electromagnetic radiation). Graeme Bartlett (talk) 21:23, 22 February 2009 (UTC)


 * OK, if we want to keep them as separate articles, we should have a good idea of what belongs where. Here's what it is now:


 *  Absorption (electromagnetic radiation) 
 * Three sentences on measuring absorption, reflection, refraction, including absorption spectroscopy.
 * Application to earth's climate.
 *  Opacity (optics) 
 * Long, quantitative discussion of light absorption and scattering.
 * Application to astrophysics, chem, pollution, and paints, including absorption spectroscopy.
 * Extinction coefficient, absorption coefficient, and their relationship.


 * I hope you'll agree that as the articles stand now, there's no logic governing which topics are in which article. But I understand that merging may not be the only way to fix that. So, what do you propose? What topics belong in Absorption that do not belong in opacity? What topics belong in Opacity that do not belong in Absorption? What belongs in both, or neither?


 * (My opinion is that there's no topic that's relevant to one but not the other, which is why I proposed merging. But I'm open to discussion.) :-) --Steve (talk) 23:40, 22 February 2009 (UTC)


 * The current situation looks very overlapping, but in the Opacity I would expect all the stuff about light, infrared and ultraviolet. Xray, Gamma ray and radio and micro wave mechanism for absorption are sufficiently different to the optical treatment that their stuff could go in the absorption article.  And that one could have an outline of the Opacity included and main article link to Opacity. Graeme Bartlett (talk) 01:46, 23 February 2009 (UTC)


 * Oppose merger with Absorption (electromagnetic radiation) article - There are multiple types of absorption spectroscopy used in chemistry to investigate multiple types of absortion by atoms, molecules, ions, etc. at various tempertatures and other conditions. Such types of absorption of photons include excitation of electron(s) from ground or lower excited state to higher excited states in both atoms and molecules, excitation by photons from lower to higher vibrational and higher rotational energy levels by bonds in molecules.  Absorption of photons by chemical reactants also causes chemical reactions to occur, and these effects and reaction quantum yields can be discussed.  Chemists seldom use the term "opacity" in connection with absorption of analytes in these kinds of spectroscopy, phenomena, or reactions.  I think much can be said in the Absorption (electromagnetic radiation) article that has not yet been said.  Photoelectric effect, photovoltaic cells, absorption of microwave and other radiation to heat food or other material, and absorption of gamma radiation by certain materials for radiation monitoring are examples of topics that can be covered (at least mentioned) under Absorption (electromagnetic radiation).  H Padleckas (talk) 12:57, 18 March 2009 (UTC)


 * OK, how about eliminating "opacity (optics)" and only having "absorption (electromagnetic radiation)"? I agree that it doesn't make much to use the term "opacity" as a title...the term is just as rare in physics as it is in chem. What's in this article that shouldn't be in "absorption (electromagnetic radiation)"? :-) --Steve (talk) 14:36, 18 March 2009 (UTC)
 * I also just found this article: Mathematical descriptions of opacity. H Padleckas (talk) 14:14, 20 March 2009 (UTC)


 * Yea, I wrote that and seem to remain essentially the only editor. All the less reason to have the "opacity (optics)" article, right? :-) ... --Steve (talk) 14:44, 20 March 2009 (UTC)
 * I've used Adobe Photoshop (6.0, I believe) graphics software, although not so extensively that I've become a complete expert. One of the parameters available is called Opacity, which can be set by the user to range from 0 - 100%, so that one layer of graphics can be set over another layer with partial opacity to allow both layers to be seen to some degree as deteremined by the user.  frame|center|See Opacity setting at upper right.
 * In effect here, opacity is the opposite of transparency. I have used this Opacity setting for making pictures.  It has its usefulness.  This is how I'm most familiar with the word "Opacity."  The vector drawing software Inkscape also uses the "Opacity" parameter in a similar way.  I have introduced a link to Opacity (optics) into the Inkscape article.  The word "Opacity" may be rather generic in these kinds of graphics software packages, in which case it may be worthwhile to retain an Opacity (optics) article.  I think I will ponder this situation further.  H Padleckas (talk) 00:38, 22 March 2009 (UTC)


 * The graphics-processing usage is covered in alpha compositing, which is already linked from opacity. So, OK to get rid of this article and put all the content into absorption (electromagnetic radiation)? --Steve (talk) 03:35, 12 May 2009 (UTC)
 * The Opacity (optics) article is about 11500 bytes long. Mathematical descriptions of opacity is about 13300 bytes long.  Absorption (electromagnetic radiation) is 2100 bytes long, but I think much more can be said about absorption of electromagnetic radiation as I've mentioned in my discussion above.  By the name of the topic, it would make sense to me to merge Mathematical descriptions of opacity into Opacity (optics), which would create an article about 24900 bytes long.  Ideally an article should have been less than 32000 bytes long, although this ideal limitation may have increased since I last checked.  However, there have been numerous good articles which are much longer.  A limit of 50000 bytes may be the current maximum limit in vogue.  Merging all three articles into Absorption (electromagnetic radiation) is another idea, giving an article about 27000 bytes long, so there would still be at least some room for expansion.  Would you consider merging Mathematical descriptions of opacity into Opacity (optics) or merging all three articles mentioned together into Absorption (electromagnetic radiation)?


 * There is also a rather short article called Transparency (optics) (without any math), which seems to me to be the opposite of opacity. There is also a rather short article called Absorbance, just to inform you what's around.  H Padleckas (talk) 05:58, 8 June 2009 (UTC)

Closing proposal: I've now edited this article to be much smaller and more specific. The quantitative discussion is only on the quantity called specifically "opacity", all the rest was moved to other articles, particularly absorption (electromagnetic radiation). It no longer needs merging. --Steve (talk) 14:35, 2 August 2009 (UTC)


 * Thanks, Steve. I'm glad we've "worked this out."  H Padleckas (talk) 01:09, 4 August 2009 (UTC)

Assessment comment
Substituted at 01:52, 30 April 2016 (UTC)

Partially opaque vs translucent
This is a very nice article. I just wanted to point out something I found a little confusing, or, maybe it's just a misunderstanding on my part about the distinctions between these three terms. To my understanding, "transparency" means more than just "all light gets through". Rather, it more means that the light waves get through while retaining their original curvature and vergence. In other words, not only does the light get through, but so does the "image". (I put that in quotes because the image doesn't actually form until the light waves are focused through a lens, but the waves needed to form the image pass through unaltered.) Even in the best glass, there is always some absorption, and colored glasses may only pass certain wavelengths, so all light does not necessarily have to get through to be transparent.

Then there's translucence. A translucent material, such as frosted glass, may transmit almost as much light as clear glass. The difference is that is diffuses the light, scattering the waves in all directions. In this case, like being in a fog, a material can be totally translucent, or anywhere between translucent and transparent. Like transparency, it can also have varying degrees of opacity depending on things like absorption, color, and haze.

Then there's opacity. An object like a mirror can be totally opaque, where no light gets through the metal coating, or, if the coating is thin enough, some light may get through and the rest reflects, making it partially opaque. The image still gets through, so it is also partially transparent, although different objects may be partially translucent and partially opaque. Laser dyes are another example. They're completely transparent to the desired wavelength for about a microsecond, and then they start absorbing the laser wavelength, making them partially opaque.

So, while I think it's a good article, it seems that this could probably use a little clearing up. Zaereth (talk) 02:34, 6 March 2021 (UTC)