Question

A convex lens of focal length $20cm$ produces a real image twice the size of the object. Then the distance of the object from the lens is

• A. 10 cm
• B. 20 cm
• C. 30 cm
• D. 60 cm

Answer 1

Answer: (C)

From lens formula $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\dots$ (i)
Also magnification $(m)=\frac{v}{u}\dots$ (ii)
Given, $f=20cm,v=2u$
$\therefore$ From Eqs. (i) and (ii), we get
$\frac{1}{20}=\frac{1}{2u}-\frac{1}{-u}$
$\Rightarrow \frac{1}{20}=\frac{1}{2u}+\frac{1}{u}$
$\Rightarrow u=30cm$