Question

A convex lens is in contact with a concave lens.The magnitude of yhe ratio of their focal length is 2/3.Their equivalent focal length is 30cm.What are their individual focal length in cm​

Answer 1

Answer:

Answer should be F convex = 10 cm and F concave = 15 cm

Explanation:

Take F convex as F1 and F concave as F2

Now,

F1/F2 = 2/3

F1 = 2*F2/3

Using formula,

1/Feq = 1/F1 + 1/F2

1/Feq = 3/2F2 - 1/F2 (Putting F1 from above and taking F2 as negative because concave lens has a negative focal length)

Solving it,

Feq = 2* F2

F2 = Feq/2 = 15 cm

And F1 = 10 cm