What is the difference in the meaning und usage of the words opacity and opaqueness?
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The difference between the two is more one of actual use than one of nuance. For simple meaning, the OED provides:
The earliest citation given for opaqueness in English is 1647, and that of opacity is 1611. Opacity is a French loanword, or was. It comes from
Whereas opaqueness was formed simply by adding the normal English derivational suffix -ness to the existing word opaque. Opacity has several senses and subsenses listed in the OED, while opaqueness has only the one I have provided above.
But as I said, the real difference is how often each is used relative to the other. This Google N-Gram shows that in actual printed use, opacity is favored over opaqueness by a favor of around 15 to 1:
Although these words aren’t normally made into plurals, they can be, and sometimes must be. Here the comparative euphony of opacities over the Gollum-like opaquenesses wins hands down in this other Google N-Gram:
In actual examples, here are the first three recent usage citations obtained by those N-Grams, first for opaquenesses:
And then for opacities:
In summary, I would strongly recommend using opacity over opaqueness.
Not entirely a new answer; I'd like to modify the idea provided by @tchrist:
This material is not suitable due to its opaqueness.
"Opacity" is usually used in technical contexts, and invokes a nuance of it being a measurable quantity. "Opaqueness" is normally used when speaking colloquially, and can also be used metaphorically to mean "incomprehensibility resulting from obscurity of meaning" (Mirriam-Webster). To that end, it would sound strange to give an actual measurement of "opaqueness". For example:
This sentence sounds strange.
This is correct.
I use opacity as a parameter between 0 and 100%, when describing the degree of transparency of something, most often web markup. For example,
Opaqueness is more familiar to me as word used for prose communication, rather than webpage design. Although the word is legitimate, I don't find it easy to use in context. It seems more idiomatic to describe something as "more opaque" or "less opaque", although an example of usage of would be to say something had "an aspect of opaqueness to it".
The difference in use is that you would use opacity to refer to understanding, when something is difficult to understand or obscure.
a. Obscurity of meaning; resistance to interpretation; impenetrability; an instance of this. rare before 20th cent.
the quality of lacking transparency or translucence: thinner paints need black added to increase opacity
the quality of being obscure in meaning: the difficulty and opacity in Barthes' texts
Opaqueness is the noun to the adjective opaque, and would be used to describe the extent to which something is opaque, or able to transmit light.
a. Not transmitting light, not transparent or translucent; impenetrable to sight. Also fig.
not able to be seen through; not transparent: bottles filled with a pale opaque liquid
(especially of language) hard or impossible to understand: technical jargon that was opaque to her
Used figuratively, in an understanding = seeing the light sort of way, the two words can become very similar, as shown particularly by the Oxford Dictionaries entries.
Opacity also has several technical uses. As pointed out by several posters, it is used in programming. It is wrong to imagine that language exists only for programmers though - opacity also has a technical use in physics, and in linguistics, and in photography, and in the paint industry, and probably others that I am unaware of. For these localised uses it is probably better to ask in the appropriate forums.