question_answer

When monochromatic red light is used instead of blue light in a convex lens, its focal length will:                        [AIEEE 11-05-2011]

A) increase                    

B) decrease

C) remain same                             

D) does not depend on colour of light

Correct Answer: A

Solution :

[a] \[{{\mu }_{R}}<{{\mu }_{B}}\]

\[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\]