Question

An object is placed at 30 cm in front of a convex lens of focal length 20 cm. Find the position of the image. What is the magnification of the object?
[4 MARKS]

Answer 1

Lens formula: 1 Mark
the position of the image: 2 Marks
magnification: 1 Mark

Here:Focal length, $f=+20cm$
Object distance, $u=-30cm$
Image distance, $v=?$
Using lens formula, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\therefore \frac{1}{v}=\frac{1}{f}-\frac{1}{u}$
Substituting the given values we have, $\frac{1}{v}=\frac{1}{-30}+\frac{1}{20}\Rightarrow \frac{1}{v}=\frac{1}{60}\Rightarrow v=60cm$
The value of v being positive indicates that the image is formed at 60 cm from the lens.
Magnification $=\frac{v}{u}=\frac{60}{-30}=-2$