1.

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

Answer» Correct Answer - C
`P=(1)/(f)rArr (P_(1))/(P_(2))=(f_(2))/(f_(1))=-(3)/(2)`
Let focal length of convex lens is `f_(1)=f`than focus length of concave length `f_(2)=-(3f)/(2)`
So,`(1)/(30)=(1)/(f)-(2)/(3f)`
`(1)/(30)=(3-2)/(3f)`
`f=10cm`
`f_(1)=10cm,f_(2)=-15cm`


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