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

`(1)/(F)=(1)/(f_(1)) +(1)/(f_(2)) = (1)/(40) +(1)/(-25)`
`=(5-8)/(200) =- (3)/(200)`
`P = (100)/(F(cm))D = (100)/(-200//3) =- 1.5D`