Question

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

Answer 1

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