Question

A double convex lens of focal length 15 cm is to be used as a magnifying glass. In order to produce an erect image which is 3 times magnified, the distance between the object and the lens should be

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

Answer 1

Answer: (B)

+10 cm

f $=$ focal length
u $=$ object distance
v $=$ image distance
f $=$ 15
m $=$+3 $=\frac{v}{u}=3$
u $=$ ?

v $=$ 3u
$\frac{1}{f}=\frac{1}{u}-\frac{1}{v}$
$\frac{1}{u}=\frac{1}{f}+\frac{1}{v}=\frac{v-f}{fv}$
$u=\frac{fv}{v+f}=\frac{15\times 3u}{3u+15}$
$3u^2+15u=45u$
$3u=30$
$u=10$