Question

An object is placed at a
distance of 5 cm more than the focal length $f$ of a convex lens. Its real image is formed at a distance $3f$ beyond the focus .The focal length $f$ of the
lens is:

• A. 5 cm
• B. 10 cm
• C. 15 cm
• D. 20 cm

Answer 1

The correct option is B 10 cm

Let magnitude of focal length of convex lens$=x$

Then $u=-(x-5)$ and $v=3x$

as $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}or\frac{1}{3x}-\frac{1}{-(x-5)}=\frac{1}{x}or\frac{1}{x+5}=\frac{2}{3x}or3x=2x+10\therefore x=10cm$

Hence $f=+10cm$