A convex lens of focal length f cut into parts first horizontally and then vertically. If the focal length of part A (quarter part) of the lens is  xf ,  then x is

When cut horizontally, focal length remains same as is clear from the following.   Formula:  1f=(μ−1)[1R−1(−R)]=2(μ−1)RWhen cut vertically, focal length is doubled  1f'=(μ−1)[1R−1∞]=(μ−1)R=12fCombining these factors, fA = 2f Quarter part of the lens will behave as a plano convexlens which has double the focal length of the given convex lens.