1.

A convex lens is in contact witha concave lens. The magnitude of the ration of their focallengths is 2//3. Their equivalent focal length is 30cm. What are their individual focal lengths?

Answer»

`-15,10`
`-10,15`
75,50
`-75,50`

Solution :`|(P_(1))/(P_(2))|=(2)/(3)rArr (f_(2))/(f_(1))=(2)/(3)`(i)
Focal length of their COMBINATION, `(1)/(f)=(1)/(f_(1))-(1)/(f_(2))`
`rArr (1)/(30)=(1)/(f_(1))-(1xx3)/(2f_(1))`
`rArr (1)/(30)=(1)/(f_(1))[1-(3)/(2)]=(1)/(f_(1))XX(-(1)/(2))`
`:. f_(1)=-15CM`
`:. f_(2)=(2)/(3)xxf_(1)=(2)/(3)xx15=10cm`


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