Talk:Optical power

Power in a medium

Removed from article:

Its a misconcept that Power of a lens is actually equal to inverse of focal length. Infact power of a lens depends on the refractive index of the medium. Power in a medium = (refrative index of medium) / (Focal length of lens in that medium).

The focal length of the lens in medium is found by lens makers formula, and the refractive index of medium should be known. The misconcept emerged because of the fact that refractive index of air (or vacuum) is one.

You're either confusing front and rear focal length with actual or "effective" focal length, or you are using a source that defines the terms in an odd way. The optical power is always one over the "effective" (or actual) focal length of the lens in whatever medium surrounds it. The front and rear focal lengths obey the relation you give. In the most general case, with different media on each side of the lens, the three focal lengths are all different.

My source for the above is page 7 of Greivenkamps' book. I'll add a reference to it to the article shortly.--Srleffler (talk) 03:22, 25 August 2008 (UTC)

I have to add to the above that it is very unfortunate that we traditionally use focal length to describe the focusing power of lenses. It makes sense for an ideal thin lens in air but is not very useful or intuitive for more complicated optical systems (or even for real non-thin simple lenses). It's much cleaner to think of the optical power as the important physical characteristic, and calculate distances to various cardinal points and planes as needed. In the most general case the focal length (one over the optical power) is not a physical distance at all—it characterizes the strength of the lens, but is not a distance between any of the cardinal points or planes.--Srleffler (talk) 03:44, 25 August 2008 (UTC)

I am a student who is learning for a very hard and conceptual exam called the IIT-JEE which, anyone who have written (or atleast looked at at) would know is definitely harder than any other competitive exam in the world. Having done a million problems from multiple books, i am definite that power of a lens is not independent of medium. You can device a certain diopter lens ad hope it to work like how it should in air and water. I am definite that my information is true and that power in a medium is (refractive index of medium)/(focal length og lens in that medium). Before removing my contribution this time, make sure you have referred with someone who knows ray optics well. I have already done that.

Peace —Preceding unsigned comment added by Nradam (talk • contribs) 18:07, 19 October 2008 (UTC)

You're right that the power of a lens depends on the medium surrounding it, but both the power and the focal length change, so the power is still the inverse of the focal length, at least as those terms are defined in the reference used in the article. It's possible that you are working from sources that define optical power and focal length differently. It's also possible that you are just wrong. Either way, since your contribution is uncited and is directly contradicted by a source cited in the article, it is immediately removable.

I suspect this comes down to just a difference in how the terms are defined. If we had a source that supports your claim, we could adjust the article to discuss this discrepancy. As I noted above, your claim works if one assumes that "focal length" refers to the distance from the front or rear principal plane to the corresponding focus (what Greivenkamp calls the front or rear focal length). This is an odd definition, though, since it makes "focal length" undefined if the medium is not the same on both sides of the optical system. Greivenkamp's defnition works even if the media are different on each side, and can be applied to individual surfaces as well as lenses immersed in some medium.--Srleffler (talk) 06:33, 20 October 2008 (UTC)

I'll try to find a good reference and mention it soon. Peace. —Preceding unsigned comment added by Nradam (talk • contribs) 03:14, 21 October 2008 (UTC)

I've been reading what you wrote again and thinking about it. What are you using for the "lensmaker's formula"? The typical form of this equation that one sees in textbooks explicitly gives the focal length of a lens in air:

${\frac {1}{f}}=(n-1)\left[{\frac {1}{R_{1}}}-{\frac {1}{R_{2}}}+{\frac {(n-1)d}{nR_{1}R_{2}}}\right],$

Your textbook(s) should be explicit about this. The focal length of the lens changes when the lens is immersed in another medium, and this equation says nothing about that case. (Note that the signs of the terms depend on an arbitrary sign convention—your book may have different signs from the above.)--Srleffler (talk) 04:32, 21 October 2008 (UTC)

You might be interested in the discussion at Talk:Lens (optics)#Lensmaker's equation again. There was an attempt to extend the lensmaker's equation in Lens (optics) to include the effect of a medium with n>1. After some debate, we dropped it due to lack of a reliable source that gives the equation. Hecht's book, which is commonly used in American physics courses, is a bit misleading. He makes the assumption that the surrounding medium has n=1 fairly early in his derivation, before "focal length" has been defined. When he gets to the latter, he defines it in a way that is only correct for a lens in vacuum, but does not explicitly note this fact. --Srleffler (talk) 05:14, 21 October 2008 (UTC)

Move to Refractive power

Google-Books hits (please check out the meaning of the term in the result set):

• 300,000 for "optical power"
• 37,400 for "optical power" lens
• 15,000 for "optical power" lens fibre
• 72,600 for "refractive power" lens

That is,

• use of "optical power" means P most of the time (> 10:1)
• for 1/f, "refractive power" is used more often than "optical power" (> 3:1)

Thus, the page should be moved to Refractive power and Optical power shall become a disambiguation page. Besides, I've not found any reference where "optical power" means "luminous power". -- Rainald62 (talk) 22:54, 17 February 2012 (UTC)

In optics, "optical power" is by far the dominant term for the quantity covered by this article. It would not be appropriate to move it to "refractive power". Alternative solutions may exist, if there is an actual problem that needs solving. I don't doubt that "optical power" as a synonym for Power carried by light is a more common usage overall than the usage of the term in optics, however I don't think we need an article specifically on that. Power is a general concept. There isn't really a need for an article on optical power P. There is a need for an article on power in the context of radiometry, but that is best placed where it is, at Radiant flux, since that is the term used in that field.

What articles would you disambiguate on the proposed dab page? We don't need to rename this article to have a dab page.--Srleffler (talk) 05:35, 18 February 2012 (UTC)

(Unrelated to "move", but...) Talking about "1/f" and "P" — is there a customary notation for the discussed quantity? Dioptre#Curvature uses "φ", but I don't remember seeing that elsewhere (actually don't remember seeing any other letter as well, :-) and have no books to look in). Perhaps, it would be useful to mention the usual notation somewhere in the article (even if it is really just "1/f"). — Mikhail Ryazanov (talk) 01:43, 20 March 2012 (UTC)

I'd totally vote for moving the current article to refractive power and linking optical power to radiant flux. In German language area it's strictly divided this way because the ambigous word "power" translates to either "Leistung" (the optical power) or "Stärke/Kraft" (what means strength, e.g. "Brechkraft" for refractive power).
Maybe it's even worth crating a completely new article on optical power describing it a little and linking to radiant flux as well as intensity. -- Patrick87 (talk) 02:56, 1 November 2012 (UTC)

The trouble is that the term "refractive power" is not generally used in optics. It may be in use in ophthalmology; I'm not sure. The general term in optics is "optical power", or simply "power".

How it's done in German is not relevant here. Articles on the English Wikipedia need to be titled based on common usage in English.

I don't see a need for a new article on optical power, distinct from Radiant flux and Power (physics).

I know it's irrelevant how it's done in German but I wanted to point out the ambiguity of the term and highlight a way to clear it up. I see optical power is more common than refractive power in English but consider also the curiosity happening in Power (physics)#Power in optics: The whole article is about the physical quantity power (measured in Watts). Then when it comes to the optical power there's suddenly a completely new quantity introduced, called power too, but measured in dioptres. That just doesn't fit in in my opinion and shows that the whole thing could do with some cleanup. -- Patrick87 (talk) 15:24, 1 November 2012 (UTC)

I made some minor changes to Power (physics)#Power in optics. It did put too much emphasis on the other meaning of the term, which is off-topic for the article.

It is important to understand that it is not our role here to resolve ambiguities that exist in the world, but rather to document and explain them. The word "power" has two meanings in optics. We can't fix that, and are not supposed to try. Our role is to make sure that our articles explain the meanings clearly and distinguish between them, that the articles have appopriate and meaningful titles that are as unambiguous as possible, and that readers can find the article that covers the subject they are interested in.--Srleffler (talk) 04:31, 2 November 2012 (UTC)

In my opinion that's exactly the thing at which we're doing a bad job right now. When I first came to this page I was searching for some definition of optical power (in the sense of radiant flux) to Wikilink it for nonprofessionals. I was surprised to find a completely different article and found it even more startling to only have a link to radiant flux (not even mentioning power in the caption of the article) on the page. I ended with not setting a Wikilink at all since I'm pretty sure somebody that is not familiar with the physical power of light won't benefit from the article on radiant flux. The article on Power (physics) is probably not suitable for such a case either since it's too common and has only few information regarding the power of light. -- Patrick87 (talk) 09:14, 2 November 2012 (UTC)

Suggest adding disambiguation terms such as 'magnification power' used in binoculars. ...Up the trail... (talk) 17:01, 17 November 2012 (UTC)upthetrail

Good idea.--Srleffler (talk) 23:28, 17 November 2012 (UTC)