Hello All, Guys in my recent lecture I discussed that unlike the focal length of spherical mirrors which is half the radius of curvature, the focal length of lenses depends not only on the geometry of lenses but also on the refractive indices of the media, and that here the relation f=R/2 is not correct. This means radius of curvature may not be equal to twice of focal length in case of lenses. And therefore the centre of curvature ( which is at distance of radius, from the optical centre) can be at a place other than 2F. But all textbooks shows C and 2F coinciding, this makes matter not realistic and it provides unclear message as to how they apply rules for image formation on it and how they are declaring the results like " If the Object is at 2F1 image will form at 2F2." What idea is there, behind making such ray diagrams? Any help will be highly appreciated!!!!