Question

A lens forms a virtual, diminished image of an object placed at $2 \,m$ from it. The size of image is half of the object. Which one of the following statements is correct regarding the nature and focal length of the lens?

• A. Concave, $|f|=1 \,m$
• B. Convex, $|f|=1 \,m$
• C. Concave, $|f|=2 \,m$
• D. Convex, $|f|=2 \,m$

Answer 1

Answer: (C)

$m=\frac{f}{f+u}, \frac{1}{2}=\frac{f}{f-2} $;
or $ 2 f=f-2 $ or $ f=-2\, m$
$|f|=2$ metre. Since the image is virtual as well as diminished,
therefore the lens is concave.