Thin lens is the index of refraction of the lens material, and "R" and "R" are the radii of curvature of the two surfaces. For a thin lens, "d" is much smaller than one of the radii of curvature (either "R" or "R"). In these conditions, the last term of the Lensmaker's equation becomes negligible, and the focal length of a thin lens in air can be approximated by Here "R" is taken to be positive if the first surface is convex, and negative if the surface is concave. The signs are reversed for the back surface of the lens: "R" is
Lens clock clock can be used if a dial thickness gauge is not available. To do this, the contact lens is placed concave side up on a table or other hard surface. The lens clock is then brought down on it such that the center prong contacts the lens as close to its center as possible, and the outer prongs rest on the table. The thickness of the lens is then the sagitta formula_4 in the formula above, and can be calculated from the optical power reading, if the distance between the outer prongs is known. Lens clock A lens clock is