Question

A convex lens forms an image of an object placed $20cm$ away from it at a distance of $20cm$ on the other side of the lens. If the object moves $5cm$ towards the lens, then the image will be :

• A. $5cm$ toward the lens
• B. $5cm$ away from the lens
• C. $10cm$ toward the lens
• D. $10cm$ away from the lens

Answer 1

Answer: (D)

$10cm$ away from the lens

Given, $u=-20cm$ ; $v=+20cm$

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

$\frac{1}{20}-\frac{-1}{20}=\frac{1}{f}$

$f=+10cm$

Now object is moved by $5cm$ towards the lens
So now, $u=-15cm$ ; $f=+10cm$

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

$\frac{1}{v}+\frac{1}{15}=\frac{1}{10}$

$v=+30cm$

So the image has moved $(30-20)=10cm$

Image is moved $10cm$ away from the lens.
option $D$ is correct