Question

A concave lens of 20 cm focal length forms an image 15 cm from the lens. Compute the object distance.

Answer 1

The lens formula says that-
$\frac{1}{f}$ = $\frac{1}{v}$ - $\frac{1}{u}$
where, v = image distance=-15 cm, as the image, will form behind the lens.
u = object distance = ?
f = focal length = -20 cm

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

$\frac{1}{-20}$ = $\frac{1}{-15}$ - $\frac{1}{u}$

$\frac{1}{u}$ = $\frac{1}{-20}$ - $\frac{1}{-15}$

$\frac{1}{u}$ = $\frac{1}{60}$

u = 60 cm
so after putting these values in the lens formula we will be getting u = 60cm behind the lens