Question

An object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm at a distance of 20 cm from the lens. Find the position of the image.
What is the magnification of the object? [4 MARKS]

Answer 1

Lens formula: 1 Mark
Object distance: 2 Marks
Magnification: 1 Mark

Here focal length of convex lens, $f=10$
Distance of object, $u=-20cm$
Distance of image,$v=?$
Using lens formula, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{v}=\frac{1}{f}+\frac{1}{u}$
$\frac{1}{v}=\frac{1}{10}+\frac{1}{-20}$
$\frac{1}{v}=\frac{1}{20}\Rightarrow v=20cm$
Thus, the image is formed at the distance of 20 cm from the lens.
Magnification$=\frac{v}{u}=\frac{20}{-20}=-1$