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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

(c) 60 cm
Object distance from the lens, u = -30 cm
Image distance from the lens, v = -20 cm

Using the lens formula, we get:

1f=1v-1u

1f=1-20-1-301f=1-20+1301f=-3+260=-160

∴ f = -60 cm

Therefore, the focal length of the lens is 60 cm.

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