Editing Lens (section)

==== Sign convention for radii of curvature {{math|''R''{{sub|1}}}} and {{math|''R''{{sub|2}}}} <span class="anchor" id="sign convention"></span>====
{{Main|Radius of curvature (optics)}}
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The signs of the lens' radii of curvature indicate whether the corresponding surfaces are convex or concave. The [[sign convention]] used to represent this varies,<ref>{{Cite web |title=Rule sign for concave and convex lens? |url=https://physics.stackexchange.com/questions/211345/rule-sign-for-concave-and-convex-lens |access-date=2024-10-27 |website=Physics Stack Exchange |language=en}}</ref> but in this article a ''positive'' {{mvar|R}} indicates a surface's center of curvature is further along in the direction of the ray travel (right, in the accompanying diagrams), while ''negative'' {{mvar|R}} means that rays reaching the surface have already passed the center of curvature. Consequently, for external lens surfaces as diagrammed above, {{math|''R''{{sub|1}} > 0}} and {{math|''R''{{sub|2}} < 0}} indicate ''convex'' surfaces (used to converge light in a positive lens), while {{math|''R''{{sub|1}} < 0}} and {{math|''R''{{sub|2}} > 0}} indicate ''concave'' surfaces. The reciprocal of the radius of curvature is called the [[curvature]]. A flat surface has zero curvature, and its radius of curvature is [[infinity|infinite]].