Question

Reena and Vani find a discarded plastic lens lying on the beach. The girls discuss what they learnt in Physics and argue whether the lens is a converging or diverging one. When they look through the lens, they notice that the objects are inverted.
i) If an object 25 cm in front of the lens forms an image 20 cm behind the lens, what is the focal length of the lens?
ii) Is it a converging or diverging lens?

Answer 1

Given: Reena and Vani find a discarded plastic lens lying on the beach. When they look through the lens, they notice that the objects are inverted. If an object 25 cm in front of the lens forms an image 20 cm behind the lens,

To find the focal length of the lens and find whether it is converging or diverging in nature

Solution:

According to the given criteria,

object distance, $u=-25cm$ (-ve as it is in front of the lens)

image distance, $v=+20cm$ (+ve as it is formed behind the lens)

Applying lens formula,

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

$⟹\frac{1}{f}=\frac{1}{20}-\frac{1}{-25}$

$⟹\frac{1}{f}=\frac{5+4}{100}$

$⟹f=11.11cm$ is the focal length of the lens found by the girls.

Convex lens produce real inverted images. Therefore, the lens is converging lens.