Question

In order to obtain a magnification of, - 0.75 with a convex lens of focal length 8 cm, the object should be placed : (a) at less than 8 cm (b) between 8 cm and 16 cm (c) beyond 16 cm (d) at 16 cm

Answer 1

m = $\frac{v}{u}$

-0.75 = $\frac{-4}{5}$ = = $\frac{v}{u}$

v = $\frac{-4}{5}u$

Using lense formula

$\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$

$\frac{-5}{4u}$ - $\frac{1}{u}$ = $\frac{1}{f}$

$\frac{-5-4}{4u}$ = $\frac{1}{f}$

$\frac{-9}{4u}$ = $\frac{1}{f}$

u = $\frac{-9f}{4}$ = -18cm

i.e., - sign signifies that the image is at the same side at the focus