A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is

Options

A. `-1.5 D`
B. `- 6.5 D`
C. `+6.5D`
D. `+6.67D`

Answer

A

Explanation

Focal length of the combination can be calculates as
`(1)/(F)=(1)/(f_(1))+(1)/(f_(2))implies(1)/(F)implies(1)/((+40))=(1)/((-25))+impliesF=-(200)/(3)cm`
`:.` Power of second lens
`P=(100)/(F)=(100)/(-200//3)=-1.5 D`