A lens is cut into two equal halves

(i) along the principal axis

(ii) Perpendicular to principal axis. What will be the focal length of each half?

(i) If the lens is cut along the principal axis than there will be no change in the focal length of the lens because the radius of curvature will remain the same, hence the focal length will remain unchanged.

(ii) If the lens is cut perpendicular to the principal axis than the focal length will change it will be twice the focal length of the original lens because the radius of curvature will increase and R = 2F.