Question

A convex lens has focal length $20cm$, which produces an image for times larger than object. Calculate the possible positions of objects.

Answer 1

Given,

A convex lens having focal length $20cm$

Since $\mu =\frac{v}{u}$

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

Now,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{20}=\frac{1}{-4u}-\frac{1}{u}=\frac{-5}{4u}$

So, $u=\frac{-5}{4}\times 20=-25cm$

the possible positions of objects. is $-25cm$