Question

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) ,Solve the example

Answer 1

Given conditions ⇒
Object distance (u) = 60 cm. (negative)
Image Distance (v) = 20 cm. (negative)

Now, Using the Lens formula, 

    $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
∴  $\frac{1}{f} = \frac{1}{-20} - \frac{1}{-60}$
⇒  f = -30 cm. 

Hence, the focal length of the lens is -30 cm.

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


Hope it helps.

Answer 2

The focal length of the f =-30.as f is negative in the sum therefore it is diverging lens