Optics Question 819
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?
Options:
A) -15, 10
B) -10, 15
C) 75, 50
D) -75, 50
Show Answer
Answer:
Correct Answer: A
Solution:
[a] $ \frac{|f _{1}|}{|f _{1}|}=\frac{2}{3} $
$ f _{1}: $ focal length of convex lens. $ \frac{1}{f}=\frac{1}{f _{1}}-\frac{1}{f _{2}} $
$ \Rightarrow \frac{1}{30}=\frac{1}{f _{1}}-\frac{1}{3f _{1}} $
$ f _{1}=10cm,f _{2}=-15cm $
