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

Options

A. 22 cm
B. 20 cm
C. 18 cm
D. none of these

Answer

B, C

Explanation

`-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