Question

A concave lens forms the image of an object such that the distance between the object and image is 10 cm and the magnification produced is 1/4.The focal length of the lens will be

• A. 8.6 cm
• B. 6.2 cm
• C. 10 cm
• D. - 4.4 cm

Answer 1

Answer: (D)

- 4.4 cm

$u-v=10$
$\frac{v}{u}=\frac{1}{4}$
$\Rightarrow 3v=10\Rightarrow v=\frac{10}{3}$
$\Rightarrow u=\frac{40}{3}$
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$-\frac{3}{10}+\frac{3}{40}=\frac{1}{f}$
$\Rightarrow f=-4.4cm.$