An eye specialist prescribes spectacles having combination of convex lens of focal length 40cm in contact with a concave lens of focal length 25cm. The power of this lens combination in diopters is

Options

A. `+1.5`
B. `-1.5`
C. `+6.67`
D. `-6.67`

Answer

B

Explanation

`f_(1)=+40cm` (for convex lens)
`=0.4m`
`f_(2)=-25` (for concave lens)
`=-0.25m`
Therefore, focal length `(f)` of the combination,
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))`
`=(1)/(0.40)-(1)/(0.25)=(0.25-0.4)/(0.40xx0.25)`
`=-(0.15)/(01)=-1.5D`
`rArrP=(1)/(f)=-1.5D`
(b) is the correct option.