Question

The image of an object in concave lens is formed at $ f/2 $ , where $ f $ is the focal length of the lens. Find the distance of the object from the lens

• A. $ f $
• B. $ 2f $
• C. $ f/2 $
• D. infinity

Answer 1

Answer: (A)

Given, focal length ( focal length is negative for concave lens)
$v=-\frac{f}{2}$ (image is always formed to the object side for concave lens)
To find,$u$ = ?
Using lens formula $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=-\frac{2}{f}-\frac{1}{u}$
$\frac{1}{u}=\frac{1}{f}-\frac{2}{f}$
$\frac{1}{u}=-\frac{1}{f}$
$\Rightarrow u=-f$