Optics Question 641
Question: $ f _{v} $ and $ f _{r} $ are the focal lengths of a convex lens for violet and red light respectively and $ F _{v} $ and $ F _{r} $ are the focal lengths of a concave lens for violet and red light respectively, then
[CBSE PMT 1996]
Options:
A) $ f _{v}<f _{r} $ and $ F _{v}>F _{r} $
B) $ f _{v}<f _{R} $ and $ F _{v}<F _{r} $
C) $ f _{c}>f _{r} $ and $ F _{v}>F _{r} $
D) $ f _{v}>f _{r} $ and $ F _{v}<F _{r} $
Show Answer
Answer:
Correct Answer: B
Solution:
According to lens makers formula $ \frac{1}{f}=(\mu -1)\ ( \frac{1}{R _{1}}-\frac{1}{R _{2}} )\Rightarrow \frac{1}{f}\propto (\mu -1) $
Since $ {\mu _{Red}}<{\mu _{violet}} $
$ \Rightarrow f _{v}<f _{r} $ and $ F _{v}<F _{r} $
Always keep in mind that whenever you are asked to compare (greater than or less than) u, v or f you must not apply sign conventions for comparison.
