Question

An object 60cm from a lens gives a virtual image at a distance of 20cm in front of the lens. What is the focal length of the lens? Is the lens converging or diverging? Give reasons for your answer.

Answer 1

Answer:

1/focal length = 1/image distance + 1/object distance

focal length positive means convex

negative means concave

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Answer 2

Answer:

Given:

Object distance, u = - 60 cm

Image distance, v = - 20 cm

Using lens formula,

1/v−1/u=1/f

⇒1/f=1/−20 −1/−60= −1/30cm

⇒f=−30 cm

Since, the focal length is negative, the lens is a diverging lens or a concave lens.

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