A person who can see things most clearly at a distance of `10 cm`. Requires spectacles to enable to him to see clearly things at a distance of `30 cm.` What should be the focal length of the spectacles ?

Options

A. `15 cm` (Concave)
B. `15 cm` (Concave)
C. `10cm`
D. `0`

Answer

A

Explanation

For lens `u=` wants to see `=-30cm`
and `v=` can see`=-10cm`
`:. (1)/(f)=(1)/(v)-(1)/(u)=(1)/(-10)-(1)/((-30))`
`impliesf=-15cm`