Two lenses are placed in contact with each other and the focal length of combination is `80 cm`. If the focal length of one is `20 cm`, then the power of the other will be

Options

A. 1.66 D
B. 4.00 D
C. `-1.00 D`
D. `-3.75 D`

Answer

D

Explanation

(d) Focal length of the combination,
`1/F=1/(f_(1))+1/f_(2)`
`rArr 1/80=1/20+1/(f_(2))`
`f_(2)=-80/3cm`
`therefore` Power of second lens,
`P'=100/(f_(2))=100/((-80//3))=-3.75D`