The focal length of the lens used in question 118 is

Options

A. `(D^(2)+d^(2))/(2D)`
B. `(D^(2)-d^(2))/(4D)`
C. `(D^(2)-d^(2))/(2D)`
D. `(D^(2)+d^(2))/(d)`

Answer

b.

Explanation

As `d=sqrt(D(D-4f))`
`d^(2)=D^(2)-4Df`
`f=(D^(2)-d^(2))/(4D)`