A convex lens has a focal length of 10 cm. At what distance from the lens should the object be placed so that it forms a real and inverted image 20 cm away from the lens?

The object should be placed at

- 20 cm

from the lens.
Given,
Image distance,

v = 20

Focal length of the lens,

f = 10 cm

From lens formula,

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

Where

f

v

and

u

are the focal length, image distance and object distance respectively.
On substituting the values, we get the

\Rightarrow \frac{1}{10} = \frac{1}{20} - \frac{1}{u}

\Rightarrow \frac{1}{u} = \frac{1}{20} - \frac{1}{10}

\Rightarrow \frac{1}{u} = - \frac{1}{20}

\Rightarrow u = - 20 cm

Hence, the object should be placed at

- 20 cm

from the lens.
Also Check:

A convex lens has a focal length of $10 cm$ . At what distance from the lens should the object be placed so that it forms a real and inverted image $20 cm$ away from the lens?