Question

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens?

• A. 10 cm
• B. 15 cm
• C. 20 cm
• D. 30 cm

Answer 1

The correct option is D

30 cm

Here,

Focal length of concave lens, $f=-15cm$

Image distance, $v=-10cm$

According to lens formula,

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

$\therefore \frac{1}{u}=\frac{1}{v}-\frac{1}{f}$ $\Rightarrow \frac{1}{u}=\frac{-1}{10}-\frac{1}{(-15)}=\frac{-1}{10}+\frac{1}{15}=\frac{-5}{150}$

Hence, $u=-30cm$

The negative value of u indicates that the object is placed 30 cm in front of the lens.