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?


Assume that Object distance (u) = – 60 cm

Image Distance (v) = -20 cm

Apply the Lens formula,

1/ f = (1/v) – (1/u)

Replace with the values given,

1/f = 1/(-20)-1/(-60) = 1/60 – 1/20 = (1-3)/ 60 = – 2/60 = -30

Hence, f = -30 cm.

Thus the focal length of the lens = -30 cm.

Given that the focal length is negative, the lens is diverging.

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