Optics Question 694
Question: A convex lens is 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
[IIT-JEE (Screening) 2005]
Options:
A) ? 75, 50
B) ? 10, 15
C) 75, 50
D) ? 15, 10
Show Answer
Answer:
Correct Answer: D
Solution:
$ \frac{f _{1}}{f _{2}}=\frac{2}{3} $ …..(i)
$ \frac{1}{f _{1}}-\frac{1}{f _{2}}=\frac{1}{30} $ …..(ii)
Solving equation (i) and (ii) $ f _{2}=-15cm $ (Concave) $ f _{1}=10cm $ (Convex)
