Question

An object is placed at a distance of $10cm$ from a convex lens of focal length $20cm$. Find the position and nature of the image.

Answer 1

Given:

Object distance, $u=-10cm$

FOcal lenght, $f=+20cm$ (convex lens)

To find: Image distance $v$ and image's nature.

Solution: From lens formula

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

$\frac{1}{v}-\frac{1}{-10}=\frac{1}{20}$

$\frac{1}{v}=\frac{1}{20}-\frac{1}{10}=\frac{1-2}{20}$

$\frac{1}{v}=\frac{-1}{20}$

$v=-20cm$

And magnification = $m=\frac{v}{u}=\frac{-20}{-10}=2$

Hence image formed is virtual, erect and magnified two times.