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Question
A concave lens produces an image 20 cm from the lens of an object placed 30 cm from the lens. The focal length of the lens is:
(a) 50 cm
(b) 40 cm
(c) 60 cm
(d) 30 cm
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Solution
60 cm
Object distance from the lens, u =-30 cm
Image distance from the lens, v =-20 cm
Using the lens formula, we get:
`1/f=1/v-1/u`
`1/f=1/-20-1/-30`
`1/f=1/-20+1/30`
`1/f=(-3+2)/60=1/60`
∴ f =- 60 cm
Therefore, the focal length of the lens is 60 cm.
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