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. (a)`-70 , 50`
B. (b)`-10 , 15`
C. (c)`75 , 50 `
D. (d)`-15 , 10`

Answer

D

Explanation

Let focal length of convex lens is `+f` , then length of concave would be `-(3)/(2)f` . From the given condition . `(1)/(30) = (1)/(f) - (2)/(f) = (1)/(3f) therefore f = 10 cm`
Therefore , focal length of convex lens = `+10` and that of concave lens = `-15 cm`