1.

The focal length of the lens used in question 118 isA. `(D^(2)+d^(2))/(2D)`B. `(D^(2)-d^(2))/(4D)`C. `(D^(2)-d^(2))/(2D)`D. `(D^(2)+d^(2))/(d)`

Answer» Correct Answer - b.
As `d=sqrt(D(D-4f))`
`d^(2)=D^(2)-4Df`
`f=(D^(2)-d^(2))/(4D)`


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