^Cutting a lens
(a) If a lens of focal length f is given a longitudinal cut (along aperture) into two equal halves as shown in fig. then the focal length of each half becomes 2f.
(b) If the lens is given a transverse cut (at right angles to aperture) into two halves by a plane parallel to principal axis) then the focal length of each part will remain f.
In this position aperture of prism becomes half, the intensity of the image formed also becomes half (as I ∝ A).