Question

A magnifying lens has a focal length of 100 mm. An object whose size is 16 mm is placed at some distance from the lens so that an image is formed at a distance of 25 cm in front of the lens. (a) What is the distance between the object and the lens? (b) Where should the object be placed if the image is to form at infinity?

Answer 1

f = 100 mm = 10 cm

v = -25 cm

ho = 16 mm = 1.6 cm

u = ?

(a).

Using the lens formula,

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

$\frac{1}{10}$ = $\frac{1}{-25}$ - $\frac{1}{u}$

On solving we get,

u = - 7.14 cm

(b). Object should be placed at the focus of the lens to form the image at infinity.

u = -f = -10 cm