A student can distinctly see the object upto a distance `15 cm`. He wants to see the black board at a distance of `3m` answer the following questions (a) Focal length of lens (b) power of lens

Options

A. `-4.8cm, -3.3D`
B. `-5.8 cm, -4.3D`
C. `-7.5 cm, -6.3 D`
D. `-15.8 cm, -6.3 D`

Answer

D

Explanation

`v=-15cm, u=-300cm,`
From lens formula `(1)/(f)=(1)/(v)-(1)/(u)`
`(1)/(f)=(1)/(-15)-(1)/(-300)=(-19)/(300)impliesf=(-300)/(19)=-15.8cm`
and power `P=(100)/(f)cm=(-100xx19)/(300)=-6.33D.`