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Question

An object is placed at a
distance of 5 cm more than the focal length f of a convex lens. Its real image is formed at a distance 3f beyond the focus .The focal length f of the
lens is:


A
5 cm
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B
10 cm
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C
15 cm
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D
20 cm
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Solution

The correct option is B 10 cm
Let magnitude of focal length of convex lens=x
Then u=(x5) and v=3x
as 1v1u=1for13x1(x5)=1xor1x+5=23xor3x=2x+10x=10cm
Hence f=+10cm

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