Optics Question 699
Question: The focal length of the field lens (which is an achromatic combination of two lenses) of telescope is 90 cm. The dispersive powers of the two lenses in the combination are 0.024 and 0.036. The focal lengths of two lenses are
[CPMT 2005]
Options:
A) 30 cm and 60 cm
B) 30 cm and ? 45 cm
C) 45 cm and 90 cm
D) 15 cm and 45 cm
Show Answer
Answer:
Correct Answer: B
Solution:
For achromatic combination $ \frac{f _{1}}{f _{2}}=-\frac{{\omega _{2}}}{{\omega _{1}}}=-\frac{0.036}{0.024}=-\frac{3}{2} $
and $ \frac{1}{f _{1}}-\frac{1}{f _{2}}=\frac{1}{90} $
solving above equations be get $ f _{1}=30cm,\ f _{2}=-45cm $
