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Question
How does the power of a lens change if its focal length is doubled?
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Solution
We know that
`"D"_1 = (100 " cm")/("F.L")` ...(i)
When focal length is doubled
`1"D"_2 = (100 " cm")/("2 F.L") = 50/"FL"` ...(ii)
`"D"_2/"D"_1 = 100/(2 "F.L") xx "F.L."/100 = 1/2`
`therefore "D"_2 = "D"_1/2`
∴ Power is Halved.
If the focal length of a lens is doubled, then its power gets halved.
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