Question

An object placed at a distance of 9cm from the first principal focus of a convex lens, produces a real image at a distance of 25cm from its second principal focus away from the lens. Then the focal length of the lens is:

• A. $9cm$
• B. $25cm$
• C. $15cm$
• D. $17cm$

Answer 1

The correct option is C $15cm$

From lens formula we have,

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\left(\frac{1}{(f+25)}\right)-\left(\frac{1}{-(f+9)}\right)=\frac{1}{f}{f}^2+25\times 9+34f=\left[\right(2f){]}^2+34f{f}^2=25\times 9F=5\times 3=15cm$