Question

if focal length of convex lens is f and it is cut into two equal part what is focal length of each part?

Answer 1

There can be two ways to cut a convex lens into two equal parts - one, along the principal axis and another, perpendicular to the principal axis. If the lens is cut along the principal axis, then there will be NO change in the focal length of the lens. But, if it is cut perpendicular to the principal axis, then the focal length will be twice the original one.