Question

An object is placed in front of convex lens at 77 cm and image formed due to it is 30.8 cm from this information find out the focal length of the lens

Answer 1

1/f = 1/v - 1/u

1/f = 1/30.8 - 1/-77

1/f = (5+2)/154

1/f = 7/154

f = 154/7

f = 22 cm

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Answer 2

As 1/f = 1/v - 1/u ( lens formula)

Therefore,

1/f = 1/30.8 - (-1/77)

1/f = 1/30.8 + 1/77

1/f = (5+2)/154

1/f = 7/154

1/f = 1/22

f = 22cm

Therefore the focal length of the lens is 22 cm.

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