1.

What is the focal length of lens if the image distance remain same by keeping object at 24 cm and 16 cm from lens ?

Answer»

22 CM
20 cm
18 cm
none of these

Solution :`-m=(f)/(f-u)` and `+m=(f)/(f-u.)`
but|-m|=|+m|
`THEREFORE-(f)/(f-u)=(f)/(f-u)`
`-(f)/(f-24)=(f)/(f-16)`
`therefore(1)/(24-f)=(1)/(f-16)`
`therefore24-f=f-16`
`therefore40=2f`
`thereforef=20` cm


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