When a lens of focal length f is cut in to two equal halves perpendicular to principle axis, then each part of the lens has a focal length

When a convex lens of focal length f is cut into two halves perpendicular

to the principal axis as shown

Combination focal length = f

Focal length of each part =${\text{f}}^{\text{|}}$

Then we can have

$\frac{1}{{\text{f}}^{\text{|}}}+\frac{1}{{\text{f}}^{\text{|}}}=\frac{1}{\text{f}}$

$\Rightarrow {\text{f}}^{\text{|}}=2\text{f}$