A convex lens of focal length f cut into parts first horizontally and then vertically. Find the focal length of part A of the lens, as shown:

Options

A. `f//2`
B. f
C. 4f
D. 2f

Answer

d.

Explanation

When cut horizontally, focal length remains same as is clear from the following formula:
`(1)/(f)=(mu-1)[(1)/(R)-(1)/((-R))]=(2(mu-1))/(R)` (i)
When cut vertically, focal length is doubled.
`(1)/(f^('))=(mu-1)[(1)/(R)-(1)/(oo)]=((mu-1))/(R)=(1)/(2f)`
Combining these factors,
`f^(')=2f`