Lens maker's formula is a formula corelating the power P (or focal length f) of a lens to radii of curvature of the two surfaces of the lens and the refractive index of lens material with respect to its surroundings. The formula is expressed as : P=(1)/(f)=(n-1)((1)/(R_(1))-(1)/(R_(2))) where n = refractive index of the material of lens with respect to its surroundings (ordinarily air) and R_(1) and R_(2) are the radii of curvature of two surfaces of the lens. The relation is true under all conditions but while applying it we should put values of P, f, R_(1) and R_(2) with their proper signs as per sign convention being followed by us. Let us have a bi- convex lens and let radii of curvature of both its curved surfaces is same i.e., |R_(1)|=|R_(2)|=R" (say)" How is your answer modified if the given biconvex lens is divided into two parts along a section XX' along its principal axis ?