Thin lens approximation for a lens is 1/v - 1/u= -1/f

Two insulated charged copper spheres A and B have their centres separated by a distance of 50 cm and have identical sizes. A third sphere of the same size but uncharged is brought in contact with the first, then brought in contact with the second, and finally removed from both. The new force of repulsion between A and B is? 5.7?10^?3/N

Magnification produced by a lens m is given by v/u

The power P of a lens is given by the approximate formula 1/f