Question

Determine how far an object must be placed in front of a converging lens of focal length 10 cm in order to produce an erect (upright) image of linear magnification 4.

Answer 1

Given:
Focal length, f = 10 cm
Magnification, m = +4 (Image is erect.)
Object distance, u = ?
Applying magnification formula, we get:
m = v/u
or, 4 = v/u
or, v = 4u
Applying lens formula, we get:
1/v $-$1/u = 1/f
1/4u $-$ 1/u = 1/10
or, u = $-$30/4
or, u = $-$7.5 cm
Thus, the object must be placed at a distance of 7.5 cm in front of the lens.