Question

A convex lens of focal 20 cm forms an image at a distance of 40 cm on the other side of the lens. Find the distance of the object from the lens. What is the magnification of the object? [4 MARKS]

Answer 1

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

f = 20 cm v = 40 cm u = ?
Using lens formula, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
or, $\frac{1}{u}=\frac{1}{v}-\frac{1}{f}$
Substituting given values we get, $\frac{1}{u}=\frac{1}{40}-\frac{1}{20}$
$\frac{1}{u}=\frac{-1}{40}\Rightarrow u=-40cm$
Hence, the object is placed at a distance of 40 cm in front of the convex lens.

Magnification$=\frac{v}{u}=\frac{40}{-40}=-1$