power 4dA convex lens of power 4D is kept in contact with a concave lens of power 3D, the effective power of combination will be . View Solution; Knowledge Check. A convex lens of focal length 40 cmPower of the lens (P) = 4 D. Refractive Index (r) = 1.5. To Find: The radius of curvature. Calculations: The following equation determines a lens's power: where P is the power of