Question

iv. An object kept 60 cm from a lens
gives a virtual image 20 cm in front
of the lens. What is the focal length
of the lens? Is it a converging lens or
diverging lens?
(Ans: -30 cm, lens is diverging or concave)​

Answer 1

Answer:

Explanation:

Solution,

Here, we have

Object distance, u = - 60 cm

Image distance, v = - 20 cm

To Find,

Focal length, f = ?

Now, we know that,

According to Lens formula,

We have,

1/f = 1/v - 1/u

So, putting all the values, we get

1/f = 1/v - 1/u

⇒ 1/f = 1/- 20 - 1/- 60

⇒ 1/f = - (1/20 - 1/60)

⇒ 1/f = - (3 - 1/60)

⇒ 1/f = - 2/60

⇒ 1/f = - 1/30

⇒ f = - 30 cm.

Hence, the focal length is 30 cm.

Since the focal length is negative, the lens is diverging.