section_path: Construction of simple lenses/Lensmaker's equation/Sign convention for radii of curvature R and R
section_name: Sign convention for radii of curvature R and R
paragraph_text: Sign convention for radii of curvature R and R
  
  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, but in this article a positive 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 R means that rays reaching the surface have already passed the center of curvature. Consequently, for external lens surfaces as diagrammed above, R > 0 and R < 0 indicate convex surfaces (used to converge light in a positive lens), while R < 0 and R > 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 infinite.