Question

A symmetrical double convex lens is cut into two equal parts along ab if focal length of original lens is 4cm

Answer 1

1/f= 1/n*(1/R1+1/R2) (The Len's Maker's Formula)

Let us suppose that the radius of curvatures are same for both sides.

1/f=2/nR

When we cut them into halves, R2=infinity for one, and R1=infinity for the other

then 1/f'=1/nR (for each half)

Then 2/f'=1/f

=> f'=2f.

Answer 2

Your question is incomplete.

The actual question is=>

A symmetrical double convex lens is cut in two equal parts vertically. If the focal length of the original lens was 4cm then what will be the focal length of the cut lens?

Answer

Plz refer to the attachment for the explanation and derivation.

Now, according to the question=>

The focal length of the lens will be-->

2× initial(actual) focal length

=>2 × 4

=>8cm.

Hence the focal length of the cut lens will be 8cm.