Question

A convex lens of focal length 15 cm produces a magnification of + 4. The object is placed : (a) at a distance of 15 cm (b) between 15 cm and 30 cm (c) at less than 15 cm (d) beyond 30 cm

Answer 1

F = -15
Using the formula $\frac{1}{u}$ + $\frac{1}{v}$ = $\frac{1}{f}$
m = $\frac{v}{u}$
v = -4 u

Using lens formula,
$\frac{1}{u}$ - $\frac{1}{4u}$ = $\frac{1}{-15}$
$\frac{3}{4u}$ = $\frac{1}{-15}$
u = $\frac{-45}{4}$