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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)
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)
Google-Books hits (please check out the meaning of the term in the result set):
That is,
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)
Suggest adding disambiguation terms such as 'magnification power' used in binoculars. ...Up the trail... ( talk) 17:01, 17 November 2012 (UTC)upthetrail
This
level-5 vital article is rated Start-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | ||||||||||||||||||||||||
|
This article was nominated for deletion on 27 June 2014 (UTC). The result of the discussion was keep. |
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)
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)
Google-Books hits (please check out the meaning of the term in the result set):
That is,
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)
Suggest adding disambiguation terms such as 'magnification power' used in binoculars. ...Up the trail... ( talk) 17:01, 17 November 2012 (UTC)upthetrail