Question

A concave lens has a focal length of 15 cm. At what distance should the object be from the lens placed so that if forms an image 10 cm from the lens?

• A. 30 cm
• B. 60 cm
• C. 90 cm
• D. 100 cm

Answer 1

The correct option is A

30 cm

A concave lens always forms a virtual, erect image on the same side as the object.

Image distance, n = - 10 cm

Focal length, f = - 15 cm

Object distance, u = ?

Using the len's formula, $\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$ we get

$\frac{1}{-10}$ - $\frac{1}{u}$ = $\frac{1}{-15}$

u = - 30 cm

Thus, the object should be placed 30 cm on the lens.