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

Options

A. `-70 , 50`
B. `-10 , 15`
C. 75 , 50
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`