Question

An object is placed $12\text{cm}$ from a concave lens of focal length $20\text{cm}$. Determine the position of the image.

• A. $12.5\text{cm}$
• B. $-9.5\text{cm}$
• C. $-7.5\text{cm}$
• D. $11\text{cm}$

Answer 1

The correct option is C $-7.5\text{cm}$


Given:
Focal length, $f=-20\text{cm}$
object distance, $u=-12\text{cm}$

Using lens formula
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{v}-\frac{1}{(-12)}=\frac{1}{-20}$

$\frac{1}{v}=\frac{-1}{20}-\frac{1}{12}=\frac{-3-5}{60}$

$\therefore v=\frac{-60}{8}=-7.5\text{cm}$

Hence, option $\left(c\right)$ is correct.