A convex lens if in contact with concave lens. The magnitude of the ratio of their focal length is `(2)/(3)`. Their equivalent focal length is 30 cm. What are their individual focal lengths?

Options

A. `-75,50`
B. `15,-10`
C. `75,50`
D. `10,-15`

Answer

D

Explanation

`(f_1)/(f_2)=(-2)/(3)impliesf_2=-(3f_1)/(2)`
`(1)/(F)-(1)/(f_1)+(1)/(f_2)`
`(1)/(30)=(1)/(f_1)+(1)/((-3f_1)/(2))=(1)/(3f_1)`
`f_1=10cm,f_2=-15cm`