Q. When monochromatic red right is used instead of blue light in a convex lens, its focal length will
A) Does not depend on colour of light
B) Increase
C) Decrease
D) Remain same
Explanation
(b) $\frac{1}{f} = (\mu - 1)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)$
Also, by Cauchy’s formula
$\mu = A + \frac{B}{\lambda^{2}} + \frac{C}{\lambda ⁴} + \ldots.$
$\lambda_{blue} < \ \lambda_{red}$
$\mu_{blue} > \ \mu_{red}$
Hence, $f_{red} > f_{blue}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.